CO/NO reactions have been studied over two catalysts, Pt/CeOx/Al2O3 and Pt/Al2O3, under net reducing conditions. The Pt/Al2O3 catalyst deactivated with time at 400 degrees C whereas the Pt/CeOx/Al2O3 catalyst showed oscillations in the reaction rate at this temperature. The oscillations had periods of 18 min. By using Fourier transform infrared spectroscopy, an attempt was made to reveal the species that inhibits the reaction over Pt. It is suggested that CeOx provides dissociation centers for NO under conditions where NO dissociation is hindered by the inhibiting species on the Pt sites.