The combined measurement of reaction rate and oxygen activity in the catalyst by solid electrolyte potentiometry (SEP) can be used in principle to examine the concept of oxygen transfer in mixed oxides, often used to explain synergetic effects between different solid phases. This is shown in the case of copper molybdate and a Mo-V-O-x mixed oxide, using the oxidation of CO as a test reaction. Both the pure oxidic phases and a physical mixture are used in the study. The rate measurements show that the addition of Mo-V-O-x, inactive in CO oxidation, enhances the catalytic activity of CuMoO4. On the other hand, SEP measurements indicate the existence of a gradient of oxygen activity between the two phases as the driving force of oxygen transfer from the Mo-V oxide to the copper molybdate.