CO oxidation over an alumina supported copper-chromium oxide has been studied by the stepresponse method. In this study the response of the catalyst is measured after step changes in the CO, '"CO and O2 concentration in order to reveal the reaction mechanism and the concentration of active sites. Special attention was given to the behaviour of the catalyst under conditions changing from net oxidization to net reduction, as this is representative for realistic exhaust gas conditions. It was found that the catalyst goes through an oxidation/reduction cycle under these conditions. The activity of the catalyst in an oxidizing gas feed following the reducing gas feed is initially higher than the steadystate activity. The surface reaction is rate determining under oxidizing and reducing conditions, but the removal of CO> from the catalyst surface also affects the overall rate of the reaction. It is found that 27% of the surface copper is covered with CO and CO2 in a reducing reaction mixture, i.e., 5% CO and 1.5% O2 in He. This amount is 18% in an oxidizing reaction mixture. CO readsorbs rapidly at the catalyst surface, but CO*, which desorbs via a concerted reaction mechanism with 02, hardly readsorbs at the catalyst surface.