The article presents measurements of the refraction coefficient R for Armco iron and 08kp steel during the oxide film growth process under isothermal as well as non-isothermal conditions at a He-Ne laser wavelength of lambda equals 0. 633 mu m. The values of the characteristic times of minima and maxima of the function R( tau ) for steel 08kp are shown. It is established that the oxidation occurs faster under non-isothermal conditions. For presentation of the experimental data the authors use the model of a two-film system with borders between the film and between the metal and the film.