The oxygen gradient beneath the oxide film on zirconium oxidized at 850°C for times ranging from 8 to 305 hr has been measured. The technique used involves the measurement of the rate of dissolution of interference colored oxide films. Experimentally determined gradients are in good agreement with a theoretical expression involving the diffusivity, time, and oxide film thickness. Metallography and microhardness measurements demonstrated the existence of a number of “phases” in the oxygen containing metal substrate. These are believed to be associated with order‐disorder reactions. No evidence of zones of constant composition or steady‐state gradients, as reported by previous investigators, were observed.