diff --git a/.cspell.json b/.cspell.json
index c7e0ea1f1..871b55b36 100644
--- a/.cspell.json
+++ b/.cspell.json
@@ -182,6 +182,7 @@
     "sharey",
     "startswith",
     "suptitle",
+    "sympified",
     "sympify",
     "symplot",
     "theano",
diff --git a/docs/usage/sympy.ipynb b/docs/usage/sympy.ipynb
index b4b715cba..f93e42676 100644
--- a/docs/usage/sympy.ipynb
+++ b/docs/usage/sympy.ipynb
@@ -162,6 +162,56 @@
     "Math(aslatex({e: e.evaluate() for e in [rho_expr, q_expr]}))"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "Class variables and default arguments to instance arguments are also supported:"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "from __future__ import annotations\n",
+    "\n",
+    "from typing import Any, ClassVar\n",
+    "\n",
+    "\n",
+    "@unevaluated_expression\n",
+    "class FunkyPower(sp.Expr):\n",
+    "    x: Any\n",
+    "    m: int = 1\n",
+    "    default_return: ClassVar[sp.Expr | None] = None\n",
+    "    _latex_repr_ = R\"f_{{{m}}}\\left({x}\\right)\"\n",
+    "\n",
+    "    def evaluate(self) -> sp.Expr | None:\n",
+    "        if self.default_return is None:\n",
+    "            return self.x**self.m\n",
+    "        return self.default_return\n",
+    "\n",
+    "\n",
+    "x = sp.Symbol(\"x\")\n",
+    "exprs = (\n",
+    "    FunkyPower(x),\n",
+    "    FunkyPower(x, 2),\n",
+    "    FunkyPower(x, m=3),\n",
+    ")\n",
+    "Math(aslatex({e: e.doit() for e in exprs}))"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "FunkyPower.default_return = sp.Rational(0.5)\n",
+    "Math(aslatex({e: e.doit() for e in exprs}))"
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {},
diff --git a/src/ampform/sympy/_decorator.py b/src/ampform/sympy/_decorator.py
index e8501b84e..a3e42ce2c 100644
--- a/src/ampform/sympy/_decorator.py
+++ b/src/ampform/sympy/_decorator.py
@@ -133,10 +133,10 @@ def _implement_new_method(cls: type[ExprClass]) -> type[ExprClass]:
     @functools.wraps(cls.__new__)
     @_insert_args_in_signature(attr_names, idx=1)
     def new_method(cls, *args, evaluate: bool = False, **kwargs) -> type[ExprClass]:
-        attr_values, kwargs = _get_attribute_values(attr_names, *args, **kwargs)
-        attr_values = sp.sympify(attr_values)
-        expr = sp.Expr.__new__(cls, *attr_values, **kwargs)
-        for name, value in zip(attr_names, attr_values):
+        positional_args, hints = _get_attribute_values(cls, attr_names, *args, **kwargs)
+        sympified_args = sp.sympify(positional_args)
+        expr = sp.Expr.__new__(cls, *sympified_args, **hints)
+        for name, value in zip(attr_names, sympified_args):
             setattr(expr, name, value)
         if evaluate:
             return expr.evaluate()
@@ -147,7 +147,7 @@ def new_method(cls, *args, evaluate: bool = False, **kwargs) -> type[ExprClass]:
 
 
 def _get_attribute_values(
-    attr_names: tuple[str, ...], *args, **kwargs
+    cls: type[ExprClass], attr_names: tuple[str, ...], *args, **kwargs
 ) -> tuple[tuple, dict[str, Any]]:
     if len(args) == len(attr_names):
         return args, kwargs
@@ -163,6 +163,10 @@ def _get_attribute_values(
         if name in kwargs:
             attr_values.append(kwargs.pop(name))
             remaining_attr_names.pop(0)
+        elif hasattr(cls, name):
+            default_value = getattr(cls, name)
+            attr_values.append(default_value)
+            remaining_attr_names.pop(0)
     if remaining_attr_names:
         msg = f"Missing constructor arguments: {', '.join(remaining_attr_names)}"
         raise ValueError(msg)
diff --git a/tests/sympy/test_decorator.py b/tests/sympy/test_decorator.py
index e446ab5a0..b9e4209a2 100644
--- a/tests/sympy/test_decorator.py
+++ b/tests/sympy/test_decorator.py
@@ -134,6 +134,43 @@ def evaluate(self) -> sp.Expr:
     assert y_expr.doit() == 5**3
 
 
+def test_unevaluated_expression_default_argument():
+    @unevaluated_expression
+    class FunkyPower(sp.Expr):
+        x: Any
+        m: int = 1
+        default_return: ClassVar[float | None] = None
+
+        def evaluate(self) -> sp.Expr:
+            if self.default_return is None:
+                return self.x**self.m
+            return sp.sympify(self.default_return)
+
+    x = sp.Symbol("x")
+    exprs = (
+        FunkyPower(x),
+        FunkyPower(x, 2),
+        FunkyPower(x, m=3),
+    )
+    assert exprs[0].doit() == x
+    assert exprs[1].doit() == x**2
+    assert exprs[2].doit() == x**3
+    for expr in exprs:
+        assert expr.x is x
+        assert isinstance(expr.m, sp.Integer)
+        assert expr.default_return is None
+
+    half = sp.Rational(1, 2)
+    FunkyPower.default_return = half
+    assert exprs[0].doit() == half
+    assert exprs[1].doit() == half
+    assert exprs[2].doit() == half
+    for expr in exprs:
+        assert expr.x is x
+        assert isinstance(expr.m, sp.Integer)
+        assert expr.default_return is half
+
+
 def test_unevaluated_expression_callable():
     @unevaluated_expression(implement_doit=False)
     class Squared(sp.Expr):
@@ -153,3 +190,20 @@ class MySqrt(sp.Expr):
     expr = MySqrt(-1)
     assert expr.is_commutative
     assert expr.is_complex  # type: ignore[attr-defined]
+
+
+def test_unevaluated_expression_default_args():
+    @unevaluated_expression
+    class MyExpr(sp.Expr):
+        x: Any
+        m: int = 2
+
+        def evaluate(self) -> sp.Expr:
+            return self.x**self.m
+
+    expr1 = MyExpr(x=5)
+    assert str(expr1) == "MyExpr(5, 2)"
+    assert expr1.doit() == 5**2
+
+    expr2 = MyExpr(4, 3)
+    assert expr2.doit() == 4**3