A study was made on the effect of the Cu/Ce ratio and on the total load of both metals on the oxidation activity of CO with O2 using SiO2 as support. It was found that the optimum Cu/Ce ratio for a constant total load of 10% on silica (Aerosil 200) was 0.43 (w/w), which corresponded to a catalyst with 3% Cu and 7% Ce. For that ratio the highly dispersed CuO-CeO2 interface was maximized. The catalyst's activity increased with increasing the total Cu-Ce load, but it reached a maximum above which the activity kept constant due to a similar concentration of sites at the CuO-CeO2 interface. The higher surface area of the support led to a increase of CuO particles interacting with CeO2 and to a decrease of bulk CuO species not interacting with CeO2. The highly dispersed CuO-CeO2 interface was not affected by a higher available surface area explaining the similar activity in supports with different surface area.