The positron lifetime technique has been used to study the relationship between the structure of chemical vapour deposited tungsten oxide thin films and the deposition parameters. Some of the positron annihilation characteristics have been evaluated for WO3 bulk material. The positron annihilation lifetime results show that there is no saturation trapping and that a considerable part of positrons annihilate from the free state. Pick-off annihilation of orthopositronium localized in two kinds of voids is observed. The growth of deposition temperature decreases the disordered material fraction. The increase in total gas flow rate leads to a decrease of large void sizes.