Oscillation behaviour of the oxidation of CO (0.2-2.2%) in air over Pt wire coils and over Pt/Al2O3 catalysts deposited onto the coils (pellistors) has been investigated. The waveforms differ considerably between the two catalytic systems. Over unsupported Pt at 240-260 degrees C, regular oscillations were accompanied by slowly declining activity and by deposition of carbon. Over supported Pt at 110-180 degrees C, relatively complex but sustained oscillation occurred by a different mechanism. This oscillation was greatly enhanced by H-2 (0.25-1.0%), and may involve fluctuations in the concentrations of CO and H-2 around the supported catalyst.