The kinetics of anodic oxidation of zircaloy-4 have been studied in 0.1M L-ascorbic acid without and with addition of a millimole of sodium phosphate at a constant current density of 8 mA.cm-2 and at room temperature (300 K). The addition of phosphate ions found to improve the kinetics of film formation. This trend and mechanism of anodic oxide film growth is confirmed from radio-tracer techniques using labelled phosphate ions