The catalytic performance of Pd/ZnO for steam reforming of methanol (CH3OH+H2O --> CO2+3H(2)) was greatly improved by previously reducing the catalysts at higher temperatures. The original catalytic functions of metallic palladium were greatly modified as a result of the formation of PdZn alloys. Over the catalysts containing alloys, formaldehyde species formed in the reaction were suggested to be effectively attacked by water, being transformed into carbon dioxide and aa hydrogen. By contrast, the formaldehyde species decomposed selectively to carbon monoxide and hydrogen over catalysts containing metallic palladium.