Sub-stoichiometric tungsten trioxide (WO3) thin films are deposited onto the glass substrates by spray pyrolysis technique using ammonium metatungstate. Effect of solution concentration on structural, morphological, optical, electrical and NO2 sensing properties of WO3 thin films is studied. Films are polycrystalline with monoclinic crystal structure and sub-stoichiometric as observed form the XRD and XPS studies, respectively. The SEM and AFM images show micro grained structure and surface roughness increases with increase in solution concentration. The PL studies revealed that the majority of the defects are the oxygen vacancies. From XPS and PL studies it is observed that, oxygen vacancies decrease with increase in solution concentration. The dielectric constant of the films as a function of frequency is in concurrence with resistivity measurements. Films show reproducible and reversible gas response at various operating temperatures and gas concentrations. Highest sensor response (38%) towards 200 ppm NO2 concentration is observed for the film with 15 mM solution concentration at moderate operating temperature (200 degrees C). Pd sensitization enhanced gas response to 68% and improved kinetics of the sensor. Films are highly selective towards NO2 as compared with the various gases such as SO2, LPG, NH3 and H2S. (C) 2014 Elsevier B. V. All rights reserved.