Contact potential difference (CPD) and effluent O2 concentration measurements were used to monitor oscillations in the surface state and reaction rate during the oxidation of hydrogen over a nickel plate in a continuous-flow reactor. Oscillatory states were observed over a wide range of temperatures (160–400 °C) for mixtures containing a large excess of hydrogen. CPD measurements indicate that the system oscillates between a state in which oxygen is chemisorbed on a reduced surface and an oxidized surface state. Periodic oscillations were observed when either significant mass or heat transfer limitations existed. Aperiodic (chaotic) behavior existed when the transport resistances were small. The characteristics of the oscillatory states became independent of linear velocity and residence time in the reactor, when the transport limitations were small, indicating that the coupling between the transport and chemical rate processes is not the cause of the oscillations.