The transport of oxidant in the scale formed during hightemperature oxidation of metals has been studied with the use of a two-stage oxidation method. This method enables the observation of the distribution of labelled oxidant in the initially formed non-labelled part of the scale. Stable isotope of oxygen,18O, and radioactive isotope of sulphur,35S, have been used in the study. Oxygen-18 has been detected with the aid of a nuclear reaction18O(p,n)18F. Transversal cross-sections of the oxidized samples have been bombarded with protons in a cyclotrone and then examined autoradiographically. Concentration changes of the isotope have been determined photometrically. It has been stated that in addition to the outward diffusion of metal, evidenced by other methods, the in ward transport of oxidant participates as a rule, in the process of scale growth. The character of oxidant transport has been suggested.