The energy dispersive (ED) variant of the conventional x-ray reflectivity (XR) provides an atomic scale determination of the morphological characteristics of thin films, such as their thickness and surface roughness. We report on the in situ EDXR measurements of the (minimal) morphological changes of ruthenium phthalocyanine gas sensing thin films. A series of reflectivity spectra have been collected, during the exposure of the films to a gas flux of nitrogen oxides (NOx) molecules. The measurements allowed a very high density time sampling of the evolution of the two morphological parameters, providing important information on the gas-film interaction. (C) 2003 American Institute of Physics.