Oxidation of alloy Ti6242S in the temperature range of 500 – 800°C in an air atmosphere is investigated by x-ray diffraction analysis, nuclear microanalysis, optical and electron microscopy. The effect of the content of oxygen at the oxide/metal interface on the lattice constants of the phases is assessed using XRD. It is shown that the concentration of oxygen in the gas-saturated layer depends on the temperature and the time of the annealing. The results obtained are expected to raise the reliability of prediction of the oxidation behavior of the alloy.