The X-ray reflectrometry technique in its energy dispersive variant (EDXR) is utilized to study the morphological characteristics of dimeric ruthenium phthalocyanine (RuPc)(2) thin films, which are being exploited as 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 of the films thickness and roughness as a consequence of the exposure to a NO2 gas flux, providing information on the gas diffusion process through the film,and on the mechanisms of interaction between film and gas molecules.