Self-sustained, isothermal oscillations in outlet species concentrations were observed under certain steady inlet conditions in the case of the CO—NO—O2—H2O reaction system on Pt/γAl2O3catalyst in a fixed-bed tubuiar reactor. The oscillations were mostly aperiodic, and for fixed inlet concentrations of the other species, they occurred in a rather narrow window of inlet oxygen concentrations near the stoichiometric point. The amplitude and frequency of these oscillations were affected by temperature and by the inlet concenlrations of CO and NO. Systemalic experiments, conducted to understand the cause of these oscillations, revealed that the catalyst aging procedure and the presence of water vapor induced the complex dynamic behavior observed in the CO—NO—O2—H2O system. The oscillations are explained qualitatively in terms of the competition among the various reactants for adsorption and subsequent reaction on the catalyst surface.