The effect of oxygen diffusion from the atmosphere on tin depth profiles in the bottom face of float glass at temperatures above the glass transition temperature (T g ) was investigated by secondary ion mass spectrometry (SIMS) with an oxygen tracer ( 1 8 O) gas and transmission electron microscopy (TEM). The heat treatment was performed in 1 8 O 2 /N 2 and argon (Ar) atmospheres. A significant movement of tin to the surface was observed for the glass heat-treated in the 1 8 O 2 /N 2 atmosphere, resulting in the formation of a tin-enriched layer near the surface region. It was found that the tin was supplied from the region shallower than the `hump' which is commonly observed in tin profiles of float glass. No significant change in the tin depth profile was observed for the glass heat-treated in Ar atmosphere. These results indicate that 1 8 O diffusion into the glass, which causes the change in chemical state of tin from Sn 2 + to Sn 4 + , induces a significant diffusion of tin. Furthermore, the precipitation of crystalline SnO 2 particles with a diameter of ~1 nm was clearly recognized in the tin-enriched layer. This fact indicates that a phase separation was induced by the oxygen diffusion into the glass. Consequently, Sn 2 + may be supplied to the surface in order to compensate for the marked decrease in Sn 2 + concentration in the glass system. It was considered that the oxidation of Sn 2 + ions into Sn 4 + in the tin-rich layers by O 2 -diffusion from the atmosphere leads to the phase separation, and the resulting decrease in the Sn 2 + concentration in the top layers that induces the out-diffusion of Sn 2 + ions from the inside.