The transient response of a high-temperature monolith catalytic combustor was studied under various conditions. The aim of the study was to elucidate the significance of the catalyst porosity on the catalyst light-off. Numerical experiments were carried out using a one-dimensional mathematical model, which accounts for both heterogeneous and homogeneous reactions and allows heat conduction in the solid phase in the axial direction. The most significant feature of the model is that it considers the reactions on the interior surface as well as on the exterior surface of the catalyst. This allows us to study the significance of the catalyst porosity on transient behavior. The effect of the porosity was studied by varying the interior surface area, the washcoat thickness and the effective diffusivity. Each of these parameters is a measure of the effect of the catalyst porosity, directly or indirectly. The results of this study show that all of these parameters have an effect on the transient performance of the catalytic combustor except when the catalyst is preheated to a very high temperature. Thus, it is concluded that larger catalyst surface area will benefit catalytic ignition. The results also show that the effect is more important at transient state than at steady state and that it depends on the type of fuel used for the combustion.