We report a study on effect of film thickness on NO2 sensing properties of sprayed WO3 thin films. WO3 thin films varying in thicknesses are deposited onto the glass substrates by simple spray pyrolysis technique by varying the volume of spray solution. Thin film gas sensors are characterized by using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), atomic force microscopy (AFM) and photoluminescence (PL) techniques to study their physical properties. Film having thickness 745 nm has shown highest gas response of 97% with 12 and 412 s response and recovery times, respectively towards 100 ppm NO2 concentration. Gas response of 20% is observed towards 10 ppm NO2 at 200 degrees C operating temperature. Sensitivity of the optimal sensor is 0.83%/ppm when operating at 200 degrees C with 10 ppm lower detection limit. The response of the sensor is reproducible and WO3 films are highly selective towards NO2 in presence of mist of various interfering gases viz. H2S, NH3, LPG, CO and SO2. (C) 2015 Elsevier Inc. All rights reserved.