Electrical sensitivity to oxidizing gases of sputtered tungsten trioxide (WO 3 ) thin films are investigated by measuring the electrical conductivity versus ozone concentration in a range from 0 to 0.8ppm at different operating temperatures. At 523K (250 o C), the resistivity is divided by a factor close to 10 3 when the ozone concentration decreases from 0.8ppm to 0. The charge transport mechanisms depend on the adsorption-desorption balance of the atmospheric gases (oxygen and ozone) and on the reactivity of the WO 3 thin-layer surface. A first (analytical) model takes into account the adsorption effect to determine the electrical charge located at the surface of each grain. A second model of microcrystalline porous WO 3 thin film derived from the theory of semiconductor devices is developed to compute the conductivity of the layer. The effects of the two mechanisms involved are analysed and a comparison between simulation and experiment is done.