The thermal oxidation of Si using H2O-O2 and H2O-N2 ambients has been studied with an automated ellipsometer which can observe the oxidation in situ. The oxidations were carried out in the temperature range of 780°-980°C on <100> oriented single crystal Si. The resulting SiO2 film growth data was analyzed according to a linear-parabolic oxidation model. The parabolic rate constant was found to increase abruptly with small additions of H2O to O2 while the linear rate constant increased gradually over the range of added H2O (0-2000 ppm). The over-all increase in the rate of oxidation due to H2O in O2 was found to be greater than predicted based on the independent diffusion and reaction of O2 and H2O related oxidant species. These effects of H2O were found to be reversible. Therefore, the kinetic role of H2O on the oxidation of silicon is essentially twofold. The H2O acts both as an additional source of oxidant and as an accelerator for the oxidation process involving O2. It is postulated that this latter effect occurs because the H2O modifies the SiO2 network thereby enhancing diffusion of the primary oxidant through the SiO2 film. © 1977, The Electrochemical Society, Inc. All rights reserved.