Carbon-supported Pt catalysts were prepared from H2PtCl6 or K2PtCl6 aqueous solutions. Particle size and structure after several thermal activation treatments were studied by X-Ray Diffraction (XRD), Transmission Electron Microscopy (TEM), and Cyclic Voltammetry (CV), and the results of the three techniques were compared. As the catalysts were highly dispersed on an amorphous support, a conventional XRD profile analysis for crystallite size determination could not be performed properly, because of the strong overlapping between the broad Pt peaks superposed to the halos of the amorphous phase. Thus, a new procedure of whole XRD pattern fitting, based on the Rietveld method, was used to have reliable data of Pt particle size (surface area) and lattice parameter. All structural and microstructural parameters were refined within the same procedure, also considering the transparency of the carbon supported catalysts and minimizing the effect of the amorphous background. The method can also take into account the presence of bimodal particle size distributions, which is difficult to study by CV or TEM.