Catalytic methane steam reforming was conducted at low temperature using a Pd catalyst supported on Ce_1−xM_xO₂ (x = 0 or 0.1, M = Ca, Ba, La, Y or Al) oxides with or without an electric field (EF). The effects of the catalyst support on catalytic activity and surface proton hopping were investigated. Results show that Pd/Al-CeO₂ (Pd/Ce_0.9Al_0.1O₂) showed higher activity than Pd/CeO₂ with EF, although their activity was identical without EF. Thermogravimetry revealed a larger amount of H₂O adsorbed onto Pd/Al-CeO₂ than onto Pd/CeO₂, so Al doping to CeO₂ contributes to greater H₂O adsorption. Furthermore, electrochemical conduction measurements of Pd/Al-CeO₂ revealed a larger contribution of surface proton hopping than that for Pd/CeO₂. This promotes the surface proton conductivity and catalytic activity during EF application.