We study the effect of the substrate material (ceramic versus metallic) on the steady-state and transient performance of monolith reactors using a one-dimensional two-phase model with position dependent transfer coefficients. When the operation of the reactor is on the ignited branch, it is shown that monoliths with metallic substrate clearly lead to a superior steady-state performance compared to those with ceramic substrate. In such cases, the ignited branch extends to lower inlet gas temperatures (typically 60-100 degrees C in after-treatment applications) for the same catalyst loading. For transient operation where the time to light-off is important, it is shown that for the case of back-end ignition (corresponding to low inlet temperatures or low catalyst loading), metallic substrates are again superior. However, for the case of front-end ignition, ceramic converters may lead to lower cumulative emissions at lower inlet gas velocities while the converse is true at higher velocities. It is shown that the transient heating time and hence the cumulative emissions decrease with decrease in the channel hydraulic diameter and thermal capacitance of the substrate but are not monotonic with the inlet gas velocity. We present mathematical analysis and simulations to support these conclusions. Some novel results on the effect of substrate conductivity on the number of steady-states and upstream propagation of temperature fronts are also presented. (C) 2013 Elsevier Ltd. All rights reserved.