Electronic properties of electrochemically formed oxide films on Nb were studied by photocurrent and differential capacitance measurements in 0.025 M KH2PO4 + 0.025 M Na2HPO4 electrolyte, pH 6.9. Oxide films of n-type conductivity were formed galvanostatically for final potentials ranging from 4 to 230 V. Measurements were performed in two potential regions, which correspond to formation of a depleted layer of variable thickness at relatively low potentials, and to complete depletion of oxide films of electronic charge carriers at higher potentials. In the first potential region the behavior of both capacitance and photocurrent, was governed by a build up of a depleted layer of potential dependent thickness. In the second, high potential, region, which extends up to the oxide film formation potential, the photocurrent and capacitance of oxide films in most features followed the trends typical of dielectric films containing defects and traps. The photocurrent and capacitance measurements on presumably dielectric oxide films formed on Ta were staged for comparison. The capacitance-potential measurements performed in the first potential region enabled us to construct the ionized donor concentration profile across the Nb2O5 film width. The limitations on the use of the C-E profiling method for electrochemically formed oxide layers are considered.