diff --git a/src/monty/io.py b/src/monty/io.py
index f7f6fbcd..c20028ea 100644
--- a/src/monty/io.py
+++ b/src/monty/io.py
@@ -76,11 +76,12 @@ def zopen(
             stacklevel=2,
         )
 
-    # Warn against default `encoding` in text mode if `PYTHONWARNDEFAULTENCODING` is set
+    # Warn against default `encoding` in text mode if
+    # `PYTHONWARNDEFAULTENCODING` is set (PEP 597)
     if (
-        os.getenv("PYTHONWARNDEFAULTENCODING", False)
-        and "t" in mode
+        "t" in mode
         and kwargs.get("encoding", None) is None
+        and os.getenv("PYTHONWARNDEFAULTENCODING", False)
     ):
         warnings.warn(
             "We strongly encourage explicit `encoding`, "
diff --git a/tests/test_io.py b/tests/test_io.py
index 5f3f78c1..5617e847 100644
--- a/tests/test_io.py
+++ b/tests/test_io.py
@@ -426,16 +426,19 @@ def test_lzw_files(self):
         # Cannot decompress a real LZW file
         with (
             pytest.raises(gzip.BadGzipFile, match="Not a gzipped file"),
+            pytest.warns(FutureWarning, match="compress LZW-compressed files"),
             zopen(f"{TEST_DIR}/real_lzw_file.txt.Z", "rt", encoding="utf-8") as f,
         ):
             f.read()
 
     @pytest.mark.parametrize("extension", [".txt", ".bz2", ".gz", ".xz", ".lzma"])
-    def test_warnings(self, extension):
+    def test_warnings(self, extension, monkeypatch):
         filename = f"test_warning{extension}"
         content = "Test warning"
 
         with ScratchDir("."):
+            monkeypatch.setenv("PYTHONWARNDEFAULTENCODING", "1")
+
             # Default `encoding` warning
             with (
                 pytest.warns(EncodingWarning, match="use UTF-8 by default"),
@@ -443,6 +446,19 @@ def test_warnings(self, extension):
             ):
                 f.write(content)
 
+            # No encoding warning if `PYTHONWARNDEFAULTENCODING` not set
+            monkeypatch.delenv("PYTHONWARNDEFAULTENCODING", raising=False)
+
+            with warnings.catch_warnings():
+                warnings.filterwarnings(
+                    "error",
+                    "We strongly encourage explicit `encoding`",
+                    EncodingWarning,
+                )
+
+                with zopen(filename, "wt") as f:
+                    f.write(content)
+
             # Implicit text/binary `mode` warning
             warnings.filterwarnings(
                 "ignore", category=EncodingWarning, message="argument not specified"