In order to study material features and to understand possible relations with functional behavior we studied morphological, structural and sensing property changes of SnO2 thin films as a function of Mo addition and oxidation process. Mo was added either before or after annealing: in the first case Mo acts as a dopant, while in the second it is a catalyst, because it is dispersed over the surface of SnO2. Thin films have been deposited by sputtering following RGTO rules [1]. Microstructural characterizations were performed by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDXS) and microraman spectroscopy. The addition of a molybdenum layer in the tin thin films leads to deep modification in the morphological structure of the film and to the formation of mixed oxides. Electrical measurements were performed in presence of some gases of interest, such as CO, NH3, ethanol, O-3, NO2, in order to evaluate influence of Mo on thin films' sensing properties.