Process of oxidation of an alloy 45H26N33S2B2 is investigated at temperatures of 400–1150 °C on air and in vacuum (10–2 mm of mercury). Experimental curve contents of oxygen on a surface of various phases in alloy structure after high-temperature endurance of various duration are submitted. Difficult nature of process of the oxidation, connected with multiphase of structure of an alloy, is revealed. High resistance of an alloy to oxidation is established.