diff --git a/docs/source/whatsnew/releases/v0.7.1.rst b/docs/source/whatsnew/releases/v0.7.1.rst
index 969a9904..811ab533 100644
--- a/docs/source/whatsnew/releases/v0.7.1.rst
+++ b/docs/source/whatsnew/releases/v0.7.1.rst
@@ -7,7 +7,8 @@ Breaking Changes
 
 Enhancements
 ------------
-
+- Add module-level constant R_GAS = 0.0083144626 in :py:mod:`pvdeg.diffusion` to avoid
+  magic numbers in code. (:pull:`313`)
 
 Deprecations
 -------------
diff --git a/pvdeg/diffusion.py b/pvdeg/diffusion.py
index a6c8bfbd..5430e67b 100644
--- a/pvdeg/diffusion.py
+++ b/pvdeg/diffusion.py
@@ -6,6 +6,8 @@
 from pvdeg import DATA_DIR
 import numpy as np
 
+R_GAS = 0.0083144626  # [kJ/mol-K]
+
 
 def esdiffusion(
     temperature,
@@ -156,28 +158,28 @@ def _get_value(d, key, default=None):
     if Dos is None:
         Dos = _get_value(esp, "Do")
     if Eads is None:
-        Eads = _get_value(esp, "Ead") / 0.0083144626
+        Eads = _get_value(esp, "Ead") / R_GAS
     else:
-        Eads = Eads / 0.0083144626
+        Eads = Eads / R_GAS
     if Sos is None:
         Sos = _get_value(esp, "So") * press
     if Eass is None:
-        Eass = _get_value(esp, "Eas") / 0.0083144626
+        Eass = _get_value(esp, "Eas") / R_GAS
     else:
-        Eass = Eass / 0.0083144626
+        Eass = Eass / R_GAS
     # These are the encapsulant oxygen permeaiton parameters
     if Doe is None:
         Doe = _get_value(encp, "Do")
     if Eade is None:
-        Eade = _get_value(encp, "Ead") / 0.0083144626
+        Eade = _get_value(encp, "Ead") / R_GAS
     else:
-        Eade = Eade / 0.0083144626
+        Eade = Eade / R_GAS
     if Soe is None:
         Soe = _get_value(encp, "So") * press
     if Ease is None:
-        Ease = _get_value(encp, "Eas") / 0.0083144626
+        Ease = _get_value(encp, "Eas") / R_GAS
     else:
-        Ease = Ease / 0.0083144626
+        Ease = Ease / R_GAS
 
     so = Sos / Soe
     eas = Eass - Ease