A laser reflection technique is used to investigate the relaxation of SiO2film stress which occurs during the dry thermal oxidation of Si between 700 and 1000 °C. Included is a determination of the stress distribution in the oxide by two independent methods: (1) measurement on oxides of various thicknesses from 100 to 800 Å, and (2) repeated stress measurements on chemically thinned SiO2films, viz. etch‐back analysis toward the interface. Agreement is found between these experiments. These thin film stress measurements are achieved by the use of ultrathin Si substrates (75 μm). Essentially, an increase in film stress with decreasing film thickness is observed. Rapid stress relaxation is observed for all temperatures studied and is attributed to a time‐dependent oxide viscosity. The influence of these measured properties on the kinetics of Si oxidation is discussed.