An electrochemical membrane reactor, RhYSZAg, has been tested for the partial oxidation of CH4 to CO/H-2 under ’oxygen pumping’ through YSZ. An ionically transported oxygen species over the Rh anode was highly active for the partial oxidation. The reaction mode of the CH4 oxidation strongly depends on the surface state of Rh particles; a highly oxidized Rh surface accelerates complete oxidation of CH4 to CO2/H2O, while a reduced Rh surface catalyzes the partial oxidation to CO/H-2. Oxygen concentration over the Rh predominantly determines the selectivity; a high oxygen concentration leads to the complete oxidation, while an adsorbed oxygen species gives the partial oxidation.