Chemical and electrochemical formation of oxide films on Nb in aqueous oxalic, sulfuric and nitric acids, at concentrations of 0.1-0.5 M in galvanostatic mode at different current densities and up to certain formation potential values, was studied. The influence of formation potential, applied current density and chemical nature of the involved electrolyte during passivation on the stability and behaviour of oxide film capacitance, were also studied. Experimental data obtained using electrochemical impedance spectroscopy technique were modeled using corresponding equivalent electrical circuits.