The reduction of NO by CO in absence and presence of O-2 has been investigated by transient experiments at automotive cold-start conditions over Pt/Rh/CeO2/gamma -Al2O3, and derived model catalysts. A high-resolution magnetic sector mass spectrometer was used for distinguishing CO/N-2 and CO2/N2O. Mechanistic comparisons are made between the catalyst formulations. A kinetic scheme of elementary reaction steps is proposed, which highlights the various contributions of the catalyst constituents.