[en] The oxidation process of the surface of superconductors is usually employed in the fabrication of Josephson tunnel junctions. The purpose of this paper is to clarify whether the oxidation model for the surface of Nb₃Ge is equally applicable to other A15 superconductors. The oxidation mechanisms were found to be divided into three groups according to ionization energies (I /SUB p/ ) of constituent atoms. In the A15 group of superconductors (formula A₃B), case I; I /SUP A/ /SUB p/ I /SUP B/ /SUB p/ : A atoms are preferentially ionized and diffuse to the surface and then form oxides. On the other hand, B atoms are hardly or little oxidized, These cases are Nb₃Ge and V₃Si, and their native oxide layers are not proper for a tunnel barrier. An exception case is Nb₃Si, but the reason is not clear. Case II; I /SUP A/ /SUB p/ I /SUP B/ /SUB p/ : Both A and B atoms are almost equally oxidized and form a composite oxide of A and B. The case is Nb₃Sn. Case III; I /SUP A/ /SUB p/ I /SUP B/ /SUB p/ : B atoms are preferentially oxidized at first and A atoms are slowly oxidized later. This case is Nb₃Al. The native oxide is proper for a tunnel barrier