The catalytic activity of Rh supported on Y-stabilized ZrO2 for NO-CO-C3H6-O-2 reactions under stoichiometric conditions was improved by the addition of 5 mol % CeO2, whereas an excess amount of CeO2 of more than 10 mol % caused a decrease in the NO reduction activity. The improvement was ascribed to the formation of easily reducible Rh species through interaction with a Ce-Zr solid solution.