The X-ray reflectrometry technique in its energy dispersive variant (EDXR) is utilized to study themorphological characteristics of dimeric ruthenium phthalocyanine (RuPc)2 thin films, which are being exploitedas nitrogen oxide gas sensors. The advantages of the EDXR with respect to its conventional (angular dispersive)counterpart are discussed. The high sensitivity of this technique allowed us to detect minimal variations ofthe films thickness and roughness as a consequence of the exposure to a NO2 gas flux, providing informationon the gas diffusion process through the film and on the mechanisms of interaction between film and gasmolecules.