In the oxidative steam reforming of methanol (OSRM), the partial oxidation of methanol (POM) is commonly assumed to be the main reaction between oxygen and methanol. This assumption was experimentally tested with a commercial Cu/ZnO/Al2O3 catalyst, and it was found that, in fact, the combustion of methanol (COM) was the main reaction between oxygen and methanol in the OSRM. Furthermore, POM was not an independent reaction but a series of COM and steam reforming of methanol (SRM) reactions. In the presence of oxygen, COM was the main reaction, and, as oxygen was depleted, SRM started and produced hydrogen. Multiple steady states were observed during the temperature cycling of the reactor between 453 and 583 K. The multiplicity was caused by COM and was analyzed in terms of the mass and heat transfer between the catalyst particle and the surrounding gas phase. Although the reactor temperature was kept below 573 K, the upper steady state of the catalyst particle (of the two steady states) was estimated to occur at temperatures above 880 K because of the high heat of combustion and the heat transfer resistance between the catalyst and the surrounding gas phase. When the catalyst was in the upper steady state, the combustion rate was so fast that COM was completed at the reactor inlet and SRM was the main reaction in the reactor. In the lower steady state, on the other hand, the combustion rate was low, and COM was the main reaction in the reactor.