Thin films of WO3 have been deposited from a gas mixture of WF6 and H2O in the temperature range 200-700 degrees C. The microstructure of the films could be controlled by the total pressure and linear gas flow velocity. A homogeneous nucleation leading to a porous oxide film was observed at 50 Torr and low gas flow velocities. In contrast, depositions carried out at 10 Ton on sapphire (01(1) over bar 2) substrates resulted in a wide deposition window for monoclinic WO3 films. These films exhibited a microstructure with columnar grains, with each grain exhibiting an epitaxial relation towards the substrate. The observed epitaxial orientations were determined to be (100)(WO3)//(01(1) over bar 2)(substrare), (010)WO3//(01(1) over bar 2)(substrate) and (001)(WO3)//(01(1) over bar 2)(substrate) with the corresponding in-plane relationships [110](WO3)//[100](substrate), [101](WO3)//[100](substrate) and [011](WO3)//[100](substrate). The deposition rate of the WO3 was higher than 1 mu m/h also at 300 degrees C. A kinetic investigation showed that the deposition process was controlled by mass transport.