Several features of the automotive catalyst support contribute to the performance of a catalytic converter system. Certainly the very high surface area and straight and uniform channels allow for an active catalytic surface while still providing a comparatively low back pressure. Other properties of the substrate such as mass and specific heat capacity prove deleterious to the rapid attainment of high conversion efficiency.The size and shape of the channel also can have positive or negative effects, depending on the relative values of these factors, which contribute to both the back pressure and the heat/mass transfer. In turn, the mass transfer is directly related to the catalyst performance.This paper examines the inter-relationships among these substrate parameters and performance properties using both calculations from measured substrate properties and measured substrate performance properties for comparison. Specifically, heat capacity, heat and mass transfer, back pressure, catalyst performance, and expected light-off performance are examined against the backdrop of the two most significantly different substrate channel structures available, the extruded square cell ceramic and the sinusoidal cell wrapped metal configurations.