A one-dimensional solid-gas model has been used to study the light-off behavior in a reactor system. Techniques from nonlinear analysis are applied to first arrive at an analytical expression to estimate the light-off location in the reactor. The model is then extended to estimate the thickening of the light-off front in the vicinity of the light-off location due to heat diffusion effects in the solid substrate. Both these results are expressed in terms of all the important operating and design variables of the monolith reactor and also the reaction specific variables such as the kinetic parameters and heats of reaction. A specific case of a three-way catalyst (TWC) is numerically treated in detail to illustrate the application of this work. Finally, we focus on using the insight obtained from this study to suggest design guidelines to optimize the performance of automotive catalytic monolith reactors. (C) 2008 American Institute of Chemical Engineers.