Energy conservation is widely recognized as an essential part of a sustainable global energy system.1 This realization brings attention to the buildings sector, which is responsible for a large and growing part of the global use of energy.Electrochromic (EC) "smart windows" are able to vary their throughput of visible light and solar radiation by the application of a low electrical voltage and can provide energy efficiency along with indoor comfort in buildings.2 The use of amorphous WO3 thin films as cathode electrodes in transmittance-modulating EC devices have been widely researched.Switching time is one of the key performance parameters for EC materials used for smart windows.The long switching time greatly hinders its application area.In order to shorten the switching time sol-gel derived tungsten oxide (WO3) films have been deposited by dipping-coating route using polyethylene glycol (PEG) dissolved in the precursor solution, followed by thermal treatment in air.After having studied the influence of the film thickness, potential window, and extended cycling on the EC properties by means of complementary electrochemical, X-ray diffraction, and electron microscopy techniques, we find that the PEG-modified WO3 films show great EC properties, the switching time has been shortened a lot, which may be attributed to the change in micro-structural, that is, PEG addition in conjunction with a post annealing treatment may generate many mesopores.