A systematic energy dispersive x-ray reflectometry study of different ruthenium phthalocyanine (RuPc)(2) thin films was performed in order to investigate their reactivity with the oxidizing NOx gas. A preliminary ex situ analysis, consisting of the comparison between the morphological parameters of different films (before and after the exposure to the gas), was performed. It suggests that a reaction involving two different mechanisms takes place. The following in situ (while fluxing the gas) reflectometry analysis confirms this hypothesis, and clarifies the temporal evolution, also revealing that the first mechanism is limited to the film surface, while the second is a bulk diffusion process.