The phase composition of anodic oxide films on transition metals is analysed using thermodynamic calculations. The model of an oxide film represents a two-layer system MkOt/MmOn, where the outer layer MkOl is the highest oxide, and the inner layer MmOn is the lower oxide of the metal. The changes of the Gibbs energy Delta G(r) are considered for the reactions of oxidation and reduction at the MkOl/MmOn interface and it is shown that for Ti, Nb, Mo, and W the former reaction predominates, resulting in the formation of the highest oxides : TiO2, Nb2O5, MoO3, and WO3. In the case of vanadium, the reaction of reduction proceeds preferentially, indicating the possibility of the lower oxide formation, namely VO2. It is concluded that anodic films on Nb, Ti, Mo and W mainly consist of the corresponding highest oxides of these metals, and NbO2, Ti2O3, MoO2 or WO2 layers are relatively thin, while in the films on vanadium the vanadium dioxide layer may be rather thick and only a very thin film near the outer boundary is close to the V2O5 stoichiometry. These results are in agreement with the experimental data on anodic oxidation of transition metals.