diff --git a/extension.go b/extension.go
index d7016de..28e170b 100644
--- a/extension.go
+++ b/extension.go
@@ -2,6 +2,7 @@ package openapi
 
 import (
 	"encoding/json"
+	"sort"
 	"strings"
 
 	"github.com/tidwall/sjson"
@@ -134,8 +135,16 @@ func marshalExtendedJSON(dst extended) ([]byte, error) {
 
 func marshalExtendedJSONInto(data []byte, obj extended) ([]byte, error) {
 	var err error
-	for k, v := range obj.exts() {
-		data, err = sjson.SetBytes(data, k, v)
+
+	exts := obj.exts()
+	keys := make([]string, 0, len(exts))
+	for k := range exts {
+		keys = append(keys, k)
+	}
+	sort.Strings(keys)
+
+	for _, k := range keys {
+		data, err = sjson.SetBytes(data, k, exts[k])
 		if err != nil {
 			return data, err
 		}
diff --git a/operation_test.go b/operation_test.go
index 8f30d35..f648092 100644
--- a/operation_test.go
+++ b/operation_test.go
@@ -3,6 +3,7 @@ package openapi_test
 import (
 	"encoding/json"
 	"fmt"
+	"strconv"
 	"testing"
 
 	"github.com/chanced/cmpjson"
@@ -104,3 +105,18 @@ func TestOperation(t *testing.T) {
 
 	}
 }
+
+func TestExtensionSorting(t *testing.T) {
+	assert := require.New(t)
+	exp := `{"x-key1":1,"x-key2":2}`
+	for n := 0; n < 100; n++ {
+		op := new(openapi.Operation)
+		op.Extensions = make(openapi.Extensions)
+		op.Extensions.SetEncodedExtension("key1", []byte("1"))
+		op.Extensions.SetEncodedExtension("key2", []byte("2"))
+
+		marshaled, _ := json.Marshal(op)
+
+		assert.Equal(string(marshaled), exp, strconv.Itoa(n))
+	}
+}