APt ring disk electrode was used in acid solutions to study the transition from Pt oxide growth to oxygen evolution and to distinguish the rates of these two processes. When a constant current is applied to the disk electrode, the disk potential, V, initially increases linearly with time, and hence with the charge density, while a negligible current is observed at the ring. In this potential region, essentially all of the applied current is used for the growth of a Pt oxide film. Following the

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... inear V/t region, V continues to increase but now more slowly and nonlinearly with time, while the ring current initially increases sharply and then slowly approaches the value expected for 100% oxygen evo]ution at the disk electrode. Thus, the Pt oxide film continues to grow in the nonlinear V/t region even when oxygen evolution becomes the major reaction. In the nonlinear V/t region, V again increases nearly linearly with the integrated charge density for oxide film formation or with the oxide film thickness. This V/q relationship in the nonlinear V/t region is different from the V/q relationship in the linear V/t region. However, the mechanism of Pt oxide growth and the properties of the film when the 02 evolution reaction is the dominant reaction remain the same as in the initial Pt oxide growth region where O~ evolution is not significant. The distribution of potentials in the oxide film and in the inner and outer Helmholtz layers is discussed.