This paper addresses the question of whether effective diffusitivies in porous materials under reactive and nonreactive conditions are equal. Previous studies have considred the problem with first-order reactions, suggesting that the effective diffusivity is unaffected by the reaction mechanisms. However, these studies were made under the assumption of heterogeneous reaction (i.e., the reaction takes place on the solid surface). In contrast to such studies, the present work shows that the effective diffusivity is affected by homogeneous reaction (i.e., the reaction takes place in the fluid phase bulk). The results, obtained from volume-averaging methods for first-order kinetics, show that the effective diffusivity is an increasing function of the Thiele modulus. (C) 2010 Elsevier Ltd. All rights reserved.