The processes occurring consecutively during anodic oxidation of antimony in buffered phosphate solutions are followed by in situ ellipsometry. In the first stages of anodization, soluble species formation followed by formation of a highly hydrated film takes place. At higher potentials, the growth of a single layer of Sb2O3 anodic film occurs. Oxygen evolution occurring simultaneously with oxide growth is also detected. Surface chemical analysis by XPS allowed obtaining the chemical state and composition of the anodic antimony oxide films. The procedure followed in order to carry out the spectra deconvolution due to overlapping of O1s and Sb3d photoemission lines is discussed. The binding energy values obtained for O1s and Sb3d signals as well as the O/Sb atomic ratio indicates that the anodic film formed at low or high potentials is composed by Sb(III) species only. Hydration as well as phosphate ions incorporation into the film is also demonstrated.