The decomposition of N2O has been widely employed by catalytic scientists to selectively measure Cu surface areas in supported Cu catalysts. The present studies on binary Cu/ZnO catalysts show, however, that under highly reducing conditions the decomposition of N2O on the reduced surface of the ZnO component of the catalyst can contribute significantly to the overall N2O decomposition, leading to high apparent Cu surface areas, Furthermore, it is clear that the susceptibility of the oxide to reduction in such catalysts is substantially enhanced by the presence of the metal and that CO is a much more powerful reducing agent in this respect than H-2, The origin and-catalytic implications of these observations are discussed.