This paper presents an investigation into the complex interactions between catalytic combustion and CH4 steam reforming in a co-flow heat exchanger where the surface combustion drives the endothermic steam reforming on opposite sides of separating plates in alternating channel flows. To this end, a simplified transient model was established to assess the stability of a system combining H-2 or CH4 combustion over a supported Pd catalyst and CH4 steam reforming over a supported Rh catalyst. The model uses previously reported detailed surface chemistry mechanisms, and results compared favorably with experiments using a flat-plate reactor with simultaneous H-2 combustion over gamma-Al2O3-supported Pd catalyst and CH4 steam reforming over a gamma-Al2O3-supported Rh catalyst. Results indicate that stable reactor operation is achievable at relatively low inlet temperatures (400 degreesC) with H-2 combustion. Model results for a reactor with CH4 combustion indicated that stable reactor operation with reforming fuel conversion to H-2 requires higher inlet temperatures.The results indicate that slow transient decay of conversion, on the order of minutes, can arise due to loss of combustion activity from high-temperature reduction of the Pd catalyst near the reactor entrance. However, model results also show that under preferred conditions, the endothermic reforming can be sustained with adequate conversion to maintain combustion catalyst temperatures within the range where activity is high. A parametric study of combustion inlet stoichiometry, temperature, and velocity reveals that higher combustion fuel/air ratios are preferred with lower inlet temperatures (less than or equal to500 degreesC) while lower fuel/air ratios are necessary at higher inlet temperatures (600 degreesC). (C) 2003 Elsevier B.V. All rights reserved.