A systematic time-resolved energy dispersive x-ray reflectometry study was performed in situ on Ruthenium Phthalocyanine thin fims to estimate the morphological detection limits of this material as NO2 transducer and the influence of the gas concentration on the gas-film interaction mechanisms. The work validates the use of this unconventional method-based on the observation of the morphological parameters change-for evaluating the response of novel sensing materials in alternative to more standard procedures. Indeed, the morphological monitoring is shown to be sensitive to the gas concentration in a range comparable to the usual electroresistive measurements. Moreover, while the latter is only able to give the information on whether the gas is interacting with the sensor, the former is also able to discriminate among interaction processes of a different nature (in the present case the interaction limited to the film surface and the one involving the material bulk). (c) 2006 American Institute of Physics.