diff --git a/py-polars/tests/unit/dataframe/test_serde.py b/py-polars/tests/unit/dataframe/test_serde.py
new file mode 100644
index 000000000000..8f806e244801
--- /dev/null
+++ b/py-polars/tests/unit/dataframe/test_serde.py
@@ -0,0 +1,24 @@
+import pytest
+
+import polars as pl
+
+
+def test_df_serialize() -> None:
+    df = pl.DataFrame({"a": [1, 2, 3], "b": [4, 5, 6]}).sort("a")
+    result = df.serialize()
+    expected = '{"columns":[{"name":"a","datatype":"Int64","bit_settings":"SORTED_ASC","values":[1,2,3]},{"name":"b","datatype":"Int64","bit_settings":"","values":[4,5,6]}]}'
+    assert result == expected
+
+
+def test_df_write_json_deprecated() -> None:
+    df = pl.DataFrame({"a": [1, 2, 3], "b": [4, 5, 6]})
+    with pytest.deprecated_call():
+        result = df.write_json()
+    assert result == df.serialize()
+
+
+def test_df_write_json_pretty_deprecated() -> None:
+    df = pl.DataFrame({"a": [1, 2, 3], "b": [4, 5, 6]})
+    with pytest.deprecated_call():
+        result = df.write_json(pretty=True)
+    assert isinstance(result, str)