Zircaloy-4 high temperature oxidation has been studied in mixed O2 + N2 + steam atmospheres and at temperatures in the 700-950°C range, conditions relevant to Spent Fuel Pool Loss-of-Coolant or Loss-of-Cooling (SFP-LOCA) scenarios. Two types of experiments using 18O as a tracer will be presented: 1-Diffusion experiments, to investigate the role of the pre-oxide barrier formed during in-reactor normal operation; 2-Oxidation experiments in mixed 18O2 + H216O + N2 atmospheres, allowing determining the respective contributions of di-oxygen and steam to the oxidation mechanisms. The zirconia scales are examined with micro-Raman imaging. Aside from the 18O distributions, the t-ZrO2 and ZrN phase repartition were also investigated.