The cooperation of single phases in a multicomponent oxidic catalyst has been investigated by combined kinetic and potentiometric (SEP) measurements. Carbon monoxide and an aldehyde have been oxidized on CuMoO4, Mo-V-O-x and a mixture of both solids. Synergetic effects in the mixed catalyst can be explained by the flow of oxygen from one single oxide to the other. Its driving force, the difference of the oxygen activity between both phases in the mixture, is determined by SEP. The direction of the flow is dependent on the reaction catalyzed and is inversed in the examples studied.