Abstract A clean Zr surface, prepared by several cycles of heating and Ar ion sputtering, is exposed to oxygen gas under 10 −5–10 −6 Pa at room temperature (RT), and surface oxidation behavior is examined by in-situ AES measurements. Subsequent depth profiling of the oxidized sample is carried out and the oxygen diffusion coefficient in α-Zr is evaluated. All AES peaks of Zr and O are modified with increasing oxygen exposure. The changes of the AES peaks show three stages of oxidation which are attributed to (1) oxygen solution in α-Zr, (2) nucleation and growth of ZrO 2 on the surface and (3) growth of the ZrO 2 layer. Above 9000 L, the surface is completely covered with ZrO 2 and the present AES study shows no evidence of the appearance of suboxide suggested by Sen et al. and de Gonzalez et al. The depth profiling of the oxidized sample indicates coexistence of ZrO 2 and α-Zr(O) with an oxygen content of around 30 at% over a depth of several nm without any clear-cut boundary of ZrO 2 and α-Zr(O). The apparent oxygen diffusion coefficient at RT estimated using a simple model, 10 −21 m 2 s −1, is much larger than the extrapolated value, around 10 −40 m 2 s −1, from the literature at high temperatures.