The microstructure of 50 nm thick WO3 films used in surface acoustic wave (SAW) gas sensors was characterized by atomic force microscopy (AFM), X-ray diffraction (XRD) and conductivity measurements. The films were deposited by r.f magnetron sputtering onto piezoelectric yz-cut LiNbO3 substrates at 200 degrees C and then subjected to annealing treatments in air. As-deposited films are amorphous, have an r.m.s. roughness of 0.6 nm and are easily eroded with the AFM tip. Annealing above 315 degrees C causes the films to crystallize into a distorted rhenium oxide structure. This crystallization is accompanied by a decrease in film conductivity and a surface roughening due to polycrystalline grain growth. The crystalline films are more sensitive for H2S gas detection and have faster response characteristics than the amorphous films.