Secondary ion mass spectrometry was used to measure the18O profiles in thermally grown SiO2which had been treated in H2 18O steam. The oxygen isotope exchange between the water and the network proceeds rapidly at 800 °C, completely exchanging a 180‐nm film in ∼20 min. Even at 400 and 600 °C, where negligible Si oxidation occurs, the network oxygen can be completely isotopically exchanged. The exchange profiles are analyzed in terms of the competing reactions involving water molecules and the SiO2network. Similar measurements of the deuterium (D) profiles in D2O‐treated films are consistent with diffusion of molecular water and its reaction with the network to transport both hydrogen and oxygen through the thermal oxide. Our results are compared to related studies which used18O nuclear resonance analysis.