The electrochemical devices based on conductive RuO2 stabilized by ceramic oxides are the most employed in chlorine-alkali cells. Their efficiency is influenced by many variables, among which the chemical nature of the stabilizer oxide plays a considerable role. In this paper, morphology, microstructure, and electrocatalytic properties of supported RuO2-SnO2 thin films, prepared in a wide compositional range, were investigated by X-ray diffraction (XRD), scanning tunneling microscopy (STM), and cyclic voltammetry (CV). X-ray analysis indicates an immiscibility zone for the two oxides approximately in the range 20-70 atom % of Sn nominal amounts. In both solubility regions, a tetragonal phase (rutile-type) is formed. Peak profile analysis indicates a nonequiaxial crystal growth and shows that in solid solutions where RuO2 is the major component larger crystallites (about 100-160 Angstrom) are formed than in those where SnO2 is the major component (about 30 Angstrom). STM characterization reveals a rough surface with a complex microstructure: very large agglomerates, which seems to be made up of a large number of smaller units with sizes of the same order of magnitude found by XRD. Only in the sample with 70 atom % of Sn (nominal amounts) there exists regions where the small units are clearly visible as distinct particles. The CV analysis shows that the highest electrocatalytic activity is exhibited by the latter sample, which has the highest possible content of RuO2 before it forms a solid solution in SnO2 and where particles tend not to form aggregates.