diff --git a/api/logic/evm.go b/api/logic/evm.go
index d451aee7e..df59ac222 100644
--- a/api/logic/evm.go
+++ b/api/logic/evm.go
@@ -2,11 +2,42 @@ package logic
 
 import (
 	"fmt"
+	"reflect"
 
 	"github.com/ethereum/go-ethereum/accounts/abi"
 	ethCommon "github.com/ethereum/go-ethereum/common"
 )
 
+// evmPreMarshal converts v to a type that gives us the JSON serialization that we like:
+// - large integers are JSON strings instead of JSON numbers
+// - byte array types are JSON strings of base64 instead of JSON arrays of numbers
+func evmPreMarshal(v interface{}, t abi.Type) interface{} {
+	switch t.T {
+	case abi.IntTy, abi.UintTy:
+		if t.Size > 32 {
+			return fmt.Sprint(v)
+		}
+	case abi.SliceTy, abi.ArrayTy:
+		rv := reflect.ValueOf(v)
+		slice := make([]interface{}, 0, rv.Len())
+		for i := 0; i < rv.Len(); i++ {
+			slice = append(slice, evmPreMarshal(rv.Index(i).Interface(), *t.Elem))
+		}
+		return slice
+	case abi.TupleTy:
+		rv := reflect.ValueOf(v)
+		m := map[string]interface{}{}
+		for i, fieldName := range t.TupleRawNames {
+			m[fieldName] = evmPreMarshal(rv.Field(i).Interface(), *t.TupleElems[i])
+		}
+	case abi.FixedBytesTy, abi.FunctionTy:
+		c := reflect.New(t.GetType()).Elem()
+		c.Set(reflect.ValueOf(v))
+		return c.Bytes()
+	}
+	return v
+}
+
 func EVMParseData(data []byte, contractABI *abi.ABI) (*abi.Method, []interface{}, error) {
 	if len(data) < 4 {
 		return nil, nil, fmt.Errorf("data (%dB) too short to have method ID", len(data))
diff --git a/api/logic/evm_test.go b/api/logic/evm_test.go
index da6848671..5a85d3b04 100644
--- a/api/logic/evm_test.go
+++ b/api/logic/evm_test.go
@@ -1,16 +1,62 @@
 package logic
 
 import (
+	_ "embed"
 	"encoding/hex"
+	"encoding/json"
 	"math/big"
 	"testing"
 
+	"github.com/ethereum/go-ethereum/accounts/abi"
 	ethCommon "github.com/ethereum/go-ethereum/common"
 	"github.com/stretchr/testify/require"
 
 	"github.com/oasisprotocol/oasis-indexer/analyzer/evmabi"
 )
 
+//go:embed test_contracts/artifacts/Varied.json
+var artifactVariedJSON []byte
+var Varied *abi.ABI
+
+func init() {
+	type artifact struct {
+		ABI *abi.ABI
+	}
+	var artifactVaried artifact
+	if err := json.Unmarshal(artifactVariedJSON, &artifactVaried); err != nil {
+		panic(err)
+	}
+	Varied = artifactVaried.ABI
+}
+
+func TestEVMParseTypes(t *testing.T) {
+	data, err := hex.DecodeString("a85acffaffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff0000000000000000000000000000000000000000000000000000000000000001ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff000000000000000000000000000000000000000000000000000000000000000100000000000000000000000000000000000000000000000000000000000000010101010101010101010101010101010101010101010101010101010101010101000000000000000000000000010101010101010101010101010101010101010101010101010101010101010101010101010101010202020200000000000000000000000000000000000000000000000000000000000000000000000000000001000000000000000000000000000000000000000000000000000000000000000100000000000000000000000000000000000000000000000000000000000001c00000000000000000000000000000000000000000000000000000000000000200000000000000000000000000000000000000000000000000000000000000024000000000000000000000000000000000000000000000000000000000000002800000000000000000000000000000000000000000000000000000000000000001010000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000016100000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000100000000000000000000000000000000000000000000000000000000000000010000000000000000000000000000000000000000000000000000000000000001000000000000000000000000000000000000000000000000000000000000004000000000000000000000000000000000000000000000000000000000000000016100000000000000000000000000000000000000000000000000000000000000")
+	require.NoError(t, err)
+	method, args, err := EVMParseData(data, Varied)
+	require.Equal(t, Varied.Methods["test"], *method)
+	jsonExpected := []string{
+		"-1",     // int8
+		"1",      // uint8
+		"\"-1\"", // int256
+		"\"1\"",  // uint256
+		"true",   // bool
+		"\"AQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQE=\"", // bytes32
+		"\"0x0101010101010101010101010101010101010101\"",   // address
+		"\"AQEBAQEBAQEBAQEBAQEBAQEBAQECAgIC\"",             // function (uint16) external returns (uint16)
+		"[1,1]",                                            // uint16[2]
+		"\"AQ==\"",                                         // bytes
+		"\"a\"",                                            // string
+		"[1]",                                              // uint16[]
+		"{\"n\":1,\"s\":\"a\"}",                            // O
+	}
+	for i, input := range method.Inputs {
+		transducedArg := evmPreMarshal(args[i], input.Type)
+		jsonBytesArg, err1 := json.Marshal(transducedArg)
+		require.NoError(t, err1)
+		require.Equal(t, jsonExpected[i], string(jsonBytesArg))
+	}
+}
+
 func TestEVMParseData(t *testing.T) {
 	// https://explorer.emerald.oasis.dev/tx/0x1ac7521df4cda38c87cff56b1311ee9362168bd794230415a37f2aff3a554a5f/internal-transactions
 	data, err := hex.DecodeString("095ea7b3000000000000000000000000250d48c5e78f1e85f7ab07fec61e93ba703ae6680000000000000000000000000000000000000000000000003782dace9d900000")
diff --git a/api/logic/test_contracts/Varied.sol b/api/logic/test_contracts/Varied.sol
new file mode 100644
index 000000000..74458f03c
--- /dev/null
+++ b/api/logic/test_contracts/Varied.sol
@@ -0,0 +1,29 @@
+// SPDX-License-Identifier: Apache-2.0
+
+pragma solidity ^0.8.18;
+
+struct O {
+    uint16 n;
+    string s;
+}
+
+interface Varied {
+    function test(
+        int8 i8,
+        uint8 u8,
+        int256 i,
+        uint256 u,
+        bool b,
+        // not supported in go-ethereum
+        // fixed128x18 f18,
+        // ufixed128x18 uf18,
+        bytes32 b32,
+        address a,
+        function (uint16) external returns (uint16) f,
+        uint16[2] calldata xy,
+        bytes calldata buf,
+        string calldata s,
+        uint16[] calldata l,
+        O calldata o
+    ) external;
+}
diff --git a/api/logic/test_contracts/artifacts/Varied.json b/api/logic/test_contracts/artifacts/Varied.json
new file mode 100644
index 000000000..9ef25cb3c
--- /dev/null
+++ b/api/logic/test_contracts/artifacts/Varied.json
@@ -0,0 +1,142 @@
+{
+  "deploy": {
+    "VM:-": {
+      "linkReferences": {},
+      "autoDeployLib": true
+    },
+    "main:1": {
+      "linkReferences": {},
+      "autoDeployLib": true
+    },
+    "ropsten:3": {
+      "linkReferences": {},
+      "autoDeployLib": true
+    },
+    "rinkeby:4": {
+      "linkReferences": {},
+      "autoDeployLib": true
+    },
+    "kovan:42": {
+      "linkReferences": {},
+      "autoDeployLib": true
+    },
+    "goerli:5": {
+      "linkReferences": {},
+      "autoDeployLib": true
+    },
+    "Custom": {
+      "linkReferences": {},
+      "autoDeployLib": true
+    }
+  },
+  "data": {
+    "bytecode": {
+      "functionDebugData": {},
+      "generatedSources": [],
+      "linkReferences": {},
+      "object": "",
+      "opcodes": "",
+      "sourceMap": ""
+    },
+    "deployedBytecode": {
+      "functionDebugData": {},
+      "generatedSources": [],
+      "immutableReferences": {},
+      "linkReferences": {},
+      "object": "",
+      "opcodes": "",
+      "sourceMap": ""
+    },
+    "gasEstimates": null,
+    "methodIdentifiers": {
+      "test(int8,uint8,int256,uint256,bool,bytes32,address,function,uint16[2],bytes,string,uint16[],(uint16,string))": "a85acffa"
+    }
+  },
+  "abi": [
+    {
+      "inputs": [
+        {
+          "internalType": "int8",
+          "name": "i8",
+          "type": "int8"
+        },
+        {
+          "internalType": "uint8",
+          "name": "u8",
+          "type": "uint8"
+        },
+        {
+          "internalType": "int256",
+          "name": "i",
+          "type": "int256"
+        },
+        {
+          "internalType": "uint256",
+          "name": "u",
+          "type": "uint256"
+        },
+        {
+          "internalType": "bool",
+          "name": "b",
+          "type": "bool"
+        },
+        {
+          "internalType": "bytes32",
+          "name": "b32",
+          "type": "bytes32"
+        },
+        {
+          "internalType": "address",
+          "name": "a",
+          "type": "address"
+        },
+        {
+          "internalType": "function (uint16) external returns (uint16)",
+          "name": "f",
+          "type": "function"
+        },
+        {
+          "internalType": "uint16[2]",
+          "name": "xy",
+          "type": "uint16[2]"
+        },
+        {
+          "internalType": "bytes",
+          "name": "buf",
+          "type": "bytes"
+        },
+        {
+          "internalType": "string",
+          "name": "s",
+          "type": "string"
+        },
+        {
+          "internalType": "uint16[]",
+          "name": "l",
+          "type": "uint16[]"
+        },
+        {
+          "components": [
+            {
+              "internalType": "uint16",
+              "name": "n",
+              "type": "uint16"
+            },
+            {
+              "internalType": "string",
+              "name": "s",
+              "type": "string"
+            }
+          ],
+          "internalType": "struct O",
+          "name": "o",
+          "type": "tuple"
+        }
+      ],
+      "name": "test",
+      "outputs": [],
+      "stateMutability": "nonpayable",
+      "type": "function"
+    }
+  ]
+}