Computational study of NO2 sensing on the WO3 (0 0 1) surface is presented. Our ab initio calculations reveal a two-step process of NO2 detection on the WO3 surface. In a first step the NO2 molecule is dissociated at an oxygen vacancy site, but a NO molecule remains adsorbed. In a second step NO is re-oxidized into NO2 by O-2 of the surrounding air leading to the resistance increase which is experimentally observed. We also calculate the adsorption energy of NO on stoichiometric and non-stoichiometric WO3 surfaces and propose a method for the NO detection. (C) 2013 Elsevier B.V. All rights reserved.