diff --git a/tox.ini b/tox.ini
index 88ae16dea..b076fc938 100644
--- a/tox.ini
+++ b/tox.ini
@@ -38,6 +38,10 @@ passenv =
 setenv =
     _PYTEST_TOX_DEFAULT_POSARGS={env:_PYTEST_TOX_POSARGS_DOCTESTING:} {env:_PYTEST_TOX_POSARGS_LSOF:} {env:_PYTEST_TOX_POSARGS_XDIST:}
 
+    # Warn where default encoding is used
+    # https://docs.python.org/3/library/io.html#io-encoding-warning
+    PYTHONWARNDEFAULTENCODING=1
+
     # Configuration to run with coverage similar to CI, e.g.
     # "tox -e py37-coverage".
     coverage: _PYTEST_TOX_COVERAGE_RUN=coverage run -m