Tungsten trioxide is a wide band-gap n-type semiconductor that has been used as a sensing material in chemiresistive gas sensors. The microstructure and morphology are important characteristics that have a large influence on the sensitivity, selectivity, and stability of the sensor. We have produced tungsten trioxide thin films 15-600 nm thick by reactive r.f. magnetron sputtering onto r-cut sapphire substrates. The microstructure of the films was characterized by reflection high-energy electron diffraction (RHEED), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). Films with local epitaxy or randomly oriented textures were produced by controlling the substrate deposition temperature and by post-deposition annealing treatments. All films were found to be dense with low porosity. Grain boundaries were identified in the films with randomly oriented texture and these films were composed of either the monoclinic or orthorhombic crystallographic phase. No grain boundaries were found for the locally epitaxial films. These films were discontinuous during early growth, exhibited evidence of crystallographic shear planes, and had a cubic crystallographic phase.