The present study investigates the response of NO2 gas sensing devices, based on bis(phthalocyaninato)titanium thin films, by combined electrical and time-resolved energy-dispersive X-ray reflectometry (EDXR) analysis. Samples of various thicknesses were exposed to a NO2 gas flux and their electrical response was recorded, during interaction with the oxidising molecules. At the same time, the changes induced in the film thickness and roughness, produced by the "breathing like" mechanism, which characterized the diffusion of the gas in the film bulk, were monitored by EDXR. Comparing the two results, the first direct correspondence between the morphological changes and the electrical response of the sensor was found. This also demonstrates that the morphological characteristics of the films are actually related to their sensing behaviour and can therefore be used, as an alternative to the electrical response, to follow the gas-film interaction process. (C) 2007 Elsevier B.V. All rights reserved.