This paper presents the results of experiments of the methanol decomposition reaction catalyzed by a commercial Cu/ZnO/Al2O3 in the absence and presence of water. Methanol decomposition of 100% in the absence of water was obtained at 290 degreesC and a space velocity of 2 cm(3)/h g cat. At these conditions, the hydrogen yield was 1.9-2.0. Water addition to the feed increased the yield of hydrogen and reduced the formation of: dimethyl ether; methyl formate and methane. The variation of the catalyst's activity and selectivity with time, temperature and feed composition was consistent with previous studies of methanol-steam reforming and water-gas shift reaction, however, this appears to be the first study over the same catalyst of methanol decomposition and methanol-steam reforming. XPS analysis of used catalyst samples and time on-stream data showed that the Cu2+ oxidation state of copper favors methanol decomposition, and we propose that the deactivation of the catalyst is mainly caused by the change in the oxidation state of copper.