A study of the effect of the aging atmosphere on the activity of co-precipitated copper zinc oxide catalysts for the ambient temperature oxidation of carbon monoxide is described and discussed. Four aging atmospheres are reported: air, N2, H2 and CO2, and both the precipitation and the aging of the precipitate were carried out by flowing these gases through the precipitation cell at constant pH and temperature. For all atmospheres, the surface area of the final CuO-ZnO catalyst increases with aging time and, consequently, the specific activity (mol CO converted/g catalyst/h) also increases. However, the intrinsic activity (mol CO converted/m2/h) initially decreases with aging time before attaining a steady level. The highest activity catalysts were obtained using air as the aging atmosphere and TPR studies indicate that this catalyst is less readily reduced. Catalysts prepared using CO2 as the aging atmosphere have lower activity, although the surface areas of these catalysts are not markedly lower. The study demonstrates that selection of the appropriate aging atmosphere, as well as the aging time, is an important parameter for the preparation of co-precipitated catalysts.