An experimental study has been carried out on the high temperature oxidation behavior of pre-oxidized Zircaloy-4. Prior to steam oxidation, cladding tubes have been pre-oxidized in moistened oxygen environment at 425°C, up to pre-oxide layer thicknesses of 35 and 65 µm. All samples have then been subjected to isothermal high temperature steam oxidation, in the 900°C-1200°C temperature range, followed by water-quenching. Metallographic examinations and mean hydrogen content measurements have then been performed. For all tested temperatures, the weight gains measured for pre-oxidized samples are lower than those obtained for bare cladding, indicating that the pre-oxide has a protective effect against oxidation at high temperature. The results obtained on the O2+steam pre-oxidized samples are in good agreement with published data on autoclave-pre-oxidized samples. Comparison with literature data obtained on irradiated cladding shows that, at 1000°C and 1200°C, laboratory-grown unirradiated pre-oxidation layers have a stronger protective influence than a corrosion layer formed under irradiation. This difference in protectiveness may be explained by a more heterogeneous oxide microstructure and a higher intergranular porosity for in-reactor corrosion in comparison with laboratory pre-oxidation conditions.