A series of Pt-Ir catalysts prepared by the impregnation method in three different supports and different atomic concentrations were characterized by conventional transmission electron microscopy (TEM), high resolution electron microscopy (HREM), X-ray diffraction (XRD) and n-hexane contact reaction as a catalytic probe. The reaction products were grouped in linear (hydrogenolysis products), isomers (2- and 3-methilpentane), cyclic (cyclohexane) and aromatic (benzene) hydrocarbons. An increment in hydrogenolysis products was observed as the amount of Ir was increased. This observation was extreme when Al2O3 and SiO2 were used as supports, but it was not so for the case Of TiO2. From TEM results, it was concluded that the metallic particle size is a function of Ir content. From X-ray diffraction analysis, it was found that a unique crystalline phase containing atoms of Pt and Ir precipitates into the specimens under study. This phase was found to have a face-centered cubic structure and a lattice parameter whose value changes as long as the relative amounts of Pt and Ir does. A linear relationship was observed to hold between the lattice parameter of the precipitated phase and the relative concentration of Pt for specimens with metallic weights of 4 and 8% in a SiO2 support. This result may be taken as a direct experimental evidence of the fact that the crystalline structure of the phase-centered cubic phase found to precipitate in amorphous silica obeys the Vegard's law and therefore, the character of a solid solution Pt(x) Ir(1 - x), where x is the Pt molar fraction, can be assigned to the structural nature of this phase. This new phase and the changes recorded in particle size could explain the changes in selectivity and catalytic activity observed in the n-hexane contact reaction.