HT-XRD analyses were performed on Zircaloy-4 sheet samples in steam and oxygen mixtures at high temperatures in order to investigate any possible relation between zirconia phase transformation and the breakaway oxidation. The tetragonal phase fraction decreases for temperatures less than 1000°C. At 1000°C and 1030°C, breakaway oxidation occurred, however, the tetragonal phase fraction remained constant during the isothermal oxidation showing no sudden decrease that could be attributed to the onset of breakaway oxidation. These results demonstrate that the tetragonal-to-monoclinic phase transformation in the external surface of the oxide could not be correlated to the occurrence of the breakaway oxidation.