The chemical interaction between NO2 gas and dimeric ruthenium phthalocyanine (RuPc)2 (Pc=phthalocyanine ligand) films has been investigated by different techniques: UV–Visible spectroscopy, X-ray Photoelectron Spectroscopy (XPS) and Extended X-ray Absorption Fine Structure (EXAFS). The optical spectra in the Q band region (700–500 nm) registered “in situ” enabled to follow the evolution of the process in real time indicating that a two steps reaction, showing two clear isosbestic points, occurs. The first phase was essentially characterised by: (a) the rapid disappearance of the 608 and 420 nm shoulders; (b) the intensity decrease of the main absorption peak and (c) the appearance of a new adsorption band centred around 510 nm. In the second step the remarkable feature is a further lowering of the main peak with the simultaneous decrease of the new 510 nm absorption. These spectral changes suggested that a chemical reaction occurred between NO2 and ruthenium phthalocyanine with the formation of a radical species due to the macrocycle oxidation. The kinetics indicates that the adsorption of gas by the evaporated (RuPc)2 film is a complex process involving more than one independent mechanism. XPS and EXAFS spectra collected before and after gas exposure showed that the central metals (Ru) were also involved in the oxidation process. The reversibility of the process has been also tested by treating the films at different temperatures, the original optical spectrum being not completely recovered.