Tungsten trioxide (WO3) thin films have been prepared by the sol-gel process and annealed at different temperatures of 400, 500, 600, and 700 degrees C for 1 h. The morphology, microstructure crystalline structure, and composition of the films have been analyzed using scanning electron microscopy (SEM), x-ray diffraction, Rutherford backscattering spectroscopy (RBS), and x-ray photoelectron spectroscopy (XPS) techniques. The SEM analysis showed that the films annealed at 400 degrees C are smooth and uniform. However, these evolved as granular at an annealing temperature of 500 degrees C. The films annealed at still higher temperatures have two distinct grains of different shapes and sizes. The films annealed below 400 degrees C are amorphous. Annealing at 500 degrees C resulted in the films having polycrystalline structure. RES and XPS characterization have revealed that the films annealed at 400 degrees C are stoichiometric. Annealing above this temperature resulted in the films becoming off-stoichiometric. The electrical resistance of the films annealed at 500 degrees C increased 18 times when exposed to 175 ppb O-3 gas compared to the air exposure, with the response time being as short as 1-2 min.