A FT-IR and EPR spectroscopic investigation has been carried out on a series of La1-xCexCoO3 (x=0-0.15) perovskite-type catalysts, which are quite active for the reduction of NO by CO, and a mechanism has been proposed for this reaction. The first step involves the oxidation of CO by the catalyst, followed by the dissociative adsorption of NO onto the catalyst surface. Finally, adsorbed nitrogen (N-ads) yields N2O, N-2 and NCOads along three parallel paths. Thus, oxygen exchange between NO and CO seems to occur indirectly and involves a surface oxygen vacancy. The catalytic activity is decreased by the substitution of Ce4+ for La3+ in the perovskite-type structure, which reduces the mobility of bulk oxygen. Activity is partially restored for x greater than or equal to 0.05 due to the presence of a segregated cerium oxide phase.