Gas phase photocatalytic decomposition of methanol with nanocrystalline tungsten trioxide (WO3) thin films in high vacuum was investigated. WO3 thin films were prepared from a novel precursor prepared using peroxo-tungstic acid and polyethylene glycol (PEG300) in water. Uniform thin films of WO3 with different morphologies such as micro-sheets, platelets, nanorods and nanoparticles were fabricated by varying the concentration of PEG300 in the precursor solution and by optimizing other preparative parameters. Nanocrystalline thin films were obtained with 20% of PEG300 in the precursor solution and at a calcination temperature of 350 degrees C, followed by post annealing in air at 500 degrees C. Photocatalytic decomposition of gaseous methanol in high vacuum was examined with nanocrystalline WO3 thin films using a quadrupole mass spectrometer at a real-time scale under visible (400-700 nm) and UVA (300-400 nm) illumination. Methanol was first decomposed to formaldehyde via direct hole transfer mechanism. Subsequently formaldehyde was decomposed to CO and finally to CO2. As a result, the partial pressures of CH2O, CO and CO2 showed a switching phenomenon according to the ON/OFF of light illumination. A rapid decrease in the photocatalytic activity was observed due to photo-induced desorption of methanol during the initial light pulse and gradual decrease at longer times was observed because of formation of tungsten bronze. Thus, the overall process of methanol decomposition over WO3 films is complex convolution of elementary steps that involve several intermediates. (C) 2011 Elsevier B.V. All rights reserved.