We report a parametric investigation of the electrical property evolution of WO3 thin films in ozone. The effect of working temperature and ozone concentration on sensors' sensitivity has been studied by alternating current (AC) impedance spectroscopy. An electrical model of resistance and capacitor circuit was used to fit the experimental spectra. It was demonstrated that the best sensors' sensitivity is obtained in the working temperature range of 250 to 300 degrees C. A tentative interpretation of the sensors' response evolution has been made by modeling of the ozone concentration effect on the WO3 sensitive layer impedances after obtaining additional AC measurements. The contributions of the grains and grain boundaries to the overall response of the WO3 based sensors were dissociated. (C) 2009 Elsevier B.V. All rights reserved.