We present a reduced-order multimode transient model for describing temperature variations in a catalytic monolith consisting of a flow channel, a thin washcoat layer in which one or more chemical reactions occur, and a ceramic or metallic support. The model developed is accurate to first order in the transverse conduction time and is valid over a wider range of operating conditions and kinetics compared with the widely used traditional two-phase model. We provide a physical interpretation of the various effective transport coefficients appearing in the reduced-order model and show that it reduces to the commonly used two-phase model only in the limit of very slowly varying inlet conditions or very slow reactions. In the general transient reacting case, we show that the traditional heat transfer coefficient concept is not applicable because the solid fluid interfacial heat flux cannot be expressed in terms of the difference between any two phase-averaged temperatures even to leading order. We use the reduced-order model to examine the light-off behavior of the monolith and the speed and width of the temperature fronts as functions of various monolith parameters.