An investigation of the formation of ultrathin In2O3 layers by oxidation of indium films is described. Ellipsometric measurements were made in situ, first to determine the complex refractive index of vacuum-deposited indium and then to follow oxide growth upon introducing oxygen. Very small changes in the optical properties of the growing In2O3 layer could be detected, making it possible to study the complex refractive index and thickness of the In2O3 as a function of oxidation time. The absorption coefficient of the oxide was observed to decrease with increasing thickness, probably because of a corresponding small compositional change. The inverse of the oxide thickness decreased linearly with the logarithm of the oxidation time, in accordance with the Mott-Cabrera theory. © 1972.