The electrochemical behaviour and solid state properties of passive oxide layers formed on binary tin + indium alloys in alkaline aqueous solution were studied by using photocurrent spectroscopy (PCS) and electrochemical impedance spectroscopy (EIS). Results obtained from PCS and EIS measurements showed that the anodically formed tin oxides behaved as heavily doped, n-type amorphous semiconductors. From the photocurrent spectra the mobility gap energies of different passive layers were evaluated The mobility gap exhibited a remarkable dependence on the alloy composition, shifting towards higher energies as the indium content in the alloy was increased. Impedance diagrams displayed a complex frequency response and the corresponding transfer function analysis employing both parametric identification procedures and non-linear fit routines, presented several contributions. The apparent flat-band potential obtained from Mott-Schottky plots depended on alloy composition, shifting more negatively with increasing indium content. On the basis of PCS and EIS data, a tentative scheme of the valence and conduction mobility edge energies for the different passive films was postulated and its correlation with the oxide layer composition critically discussed.