The goal of work described in this paper was to better understand the methanol steam reforming (MSR) activity and selectivity patterns of ZnO and CeO2 supported Pd catalysts. This reaction is being used to produce 1-12-rich gas for a number of applications including hydrogen fuel cells. The Pd/ZnO catalysts had lower MSR rates but were more selective for the production Of CO2 than the Pd/CeO2 catalysts. The CH3OH conversion rates were proportional to the H-2 chemisorption uptake suggesting that the rate determining step was catalyzed by Pd. The corresponding turnover frequencies averaged 0.8 +/- 0.3 s(-1) and 0.4 +/- 0.2 s(-1) at 230 degrees C for the Pd/ZnO and Pd/CeO2 catalysts, respectively. The selectivities are explained based on the reaction pathways, and characteristics of the support. The key surface intermediate appeared to be a formate. The ZnO supported catalysts had a higher density of acidic sites and favored pathways where the intermediate was converted to CO2 while the CeO2 supported catalysts had a higher density of basic sites and favored the production of CO. (c) 2005 Published by Elsevier B.V.