The reaction between NO and H2 was studied over the Pt(100) single crystalsurface. Under suitably chosen parameters (partial pressures, flow rate, etc.) and under isothermal conditions, the reaction exhibited sustained temporal oscillations in reaction rate as seen by the variation in product and reactant partial pressure measured by a quadrupolemass spectrometer. The oscillations observed for the formation rates of N2 and NH3 are out of phase with those found for the formation rates of N2O. Systematic small changes in the H2 partial pressure cause the system to move from a single period oscillation via a sequence of period doublings to oscillations which were aperiodic or chaotic in nature. Analysis of this data through power spectra and correlation integrals identified it as being chaotic, and not noisy in nature, and these calculations also revealed that the minimum number of variables determining the physical properties of the system to be 3. On the basis of this and careful analysis of the different product formations, a mechanism is proposed for this oscillating system.