diff --git a/tox.ini b/tox.ini
index dc2042b..4401a38 100644
--- a/tox.ini
+++ b/tox.ini
@@ -6,9 +6,13 @@ envlist =
     py27
     py36
 
+[depversions]
+flake8 = 3.7.7
+sphinx_rtd_theme = 0.4.3
+
 [testenv:flake8]
 deps =
-    flake8
+    flake8=={[depversions]flake8}
 skip_install = True
 commands = flake8 \
     setup.py \
@@ -17,7 +21,7 @@ commands = flake8 \
 
 [testenv:docs]
 deps =
-    sphinx_rtd_theme==0.4.3
+    sphinx_rtd_theme=={[depversions]sphinx_rtd_theme}
 changedir = {toxinidir}/docs
 commands =
     sphinx-build -b html . "_build/html"