Nb2O5 thin films were deposited onto heated glass substrates by RF magnetron sputtering using a Nb2O5 target. The films were annealed in air at temperatures between 400 and 700 degrees C for 6 h. Effects of the crystalline structure on optical, structural, electrochromic and bonding properties of the Nb2O5 thin films were investigated by X-ray diffraction, atomic force microscopy, X-ray photoelectron spectroscopy, optical spectrophotometry and electrochemical measurements. The film refractive index varied between 2.09 and 2.22 at the wavelength of 550 nm depending on the annealing temperature. The decrease of the optical band gap revealed for the films with increasing annealing temperature is attributed to oxygen-ion vacancies in the film structure. The orthorhombic structure of Nb2O5 films resulted in good electrochromic properties with high colouration efficiencies of 19.56 cm(2)/C and 53.24 cm(2)/C at 550 nm and 1000 nm, respectively. The optical, structural and electrochromic properties of the different crystalline polymorphic forms of the Nb2O5 films make them attractive for optical applications. (C) 2015 Elsevier B.V. All rights reserved.