Niobium oxide films were grown in an acidic environment by application of an alternating potential to metallic niobium substrate. These anodic oxide films show interference colour (iridescence) and their electrochromic characteristics were optically and electrochemically analysed. A model where the reaction rate is dominated by the diffusion of ionic pairs in the oxide films was assumed, and it allowed the reproduction of transient dependence of the current and the absorbance. Results indicate that the model contains the essential elements of the kinetics and that the optical and electrochemical phenomena are strictly related.