Structural and stability studies of bimetallic Pt-Sn (3:1) nanoparticles were performed in situ via X-ray diffraction at wide angles (WAXS). The homemade bimetallic catalyst (Pt-Sn) ccomp (ecomp = from carbonyl complex) was synthesized in mild conditions from a Pt-carbonyl chemical precursor. A relatively narrow size distribution (2.4 +/- 0.9 nm) of such a bimetallic catalyst supported onto carbon Vulcan XC72 was obtained at room temperature. Its electrochemical behavior was compared to that of a commercial catalyst. The WAXS study revealed that such a catalyst, prepared via the carbonyl route, has a certain degree of surface disorder (high Debye parameter, B), which enhances the electrocatalytic activity for hydrogen adsorption. Furthermore, WAXS also demonstrated that the structural stability of this bimetallic catalyst is maintained at the annealing temperature employed (500 degreesC), although the particle size increases from 1.6 to 2.2 nm. Electrochemical underpotential deposition studies, via copper deposition, also provide information concerning the state of the nanoparticulate surface of the various platinum-based catalysts investigated.