A significant advantage of monolithic cellular catalytic converters is the high substrate specific surface area offered for catalyst distribution. It has been shown elsewhere that the substrate total surface area can act as a surrogate for other catalyst parameters in estimating overall catalytic performance. Lacking in the literature, however, are indications of how this surface area influence changes with aging time and temperature. Also, there has been a tacit assumption that all surface areas are equivalent and that the underlying material and cell structure play no significant role. For these reasons, aging studies were carried out on two substrate configurations (extruded square cell ceramic and wrapped foil metal) to establish the surface area influences over time at temperatures of interest to the automotive companies. It is anticipated that the results of this study will be used to more effectively design catalysts to meet increasingly demanding durability requirements.