The initial oxidation of polycrystalline alpha-Fe and polycrystalline epsilon-Fe2N1-x was investigated with ellipsometry. Prior to oxidation the sample surfaces were subjected to a sputter cleaning pretreatment or a sputter cleaning plus annealing pretreatment. For the wavelength applied in the ellipsometric analysis, i.e. 632.8 nm, the complex indices of refraction of the two cleaned substrates were practically independent of the pretreatment. The samples were subsequently oxidized at p(O2) = 10(-4) Pa at temperatures ranging from room temperature to 500 K. A procedure for the simultaneous determination of the (real) refractive index and the thickness of the oxide film was given. The effect of surface and interface roughness was analysed. No significant difference was found between the (real) refractive indices of the oxide films grown on the iron and on the iron-nitride sample, and no influence of the pretreatment was observed. The determination of the oxide-film thickness from the results of ellipsometry was discussed.