Vulcan XC-72 carbon-supported Pt-Ni alloy nanoparticle catalysts with different Pt/Ni atomic composition were prepared via the carbonyl complex route and their structure was studied by X-ray diffraction spectroscopy at wide angles (WAXS) and Debye function analysis (DFA). The very good agreement between the WAXS pattern and DFA simulation revealed that all the as-prepared Pt-Ni alloy catalysts have a unique and highly disordered face-centered cubic structure (solid solution) and that the lattice parameter decreases with the increase of the Ni content in the alloys. Transmission electron microscopy (TEM) images indicated that the as-prepared Pt-Ni alloy nanoparticles were well dispersed on the surface of the carbon support with a narrow particle size distribution and that their mean particle size slightly decreased with the increase in Ni content. Energy-dispersive X-ray analysis (EDX) confirmed that the catalyst composition was nearly the same as that of the nominal value. Thus, a comparative study was made for the oxygen reduction reaction (ORR) using the thin-film rotating ring-disk electrode method to the behavior of Pt based catalysts on the same carbon support, having the same metal loading, the same disordered structure, and a similar particle size. As compared to the Pt/C catalyst, the bimetallic catalysts with different Pt/Ni atomic ratios exhibited an enhancement factor of ca. 1.5 to 3 in the mass activity and of ca. 1.5 to 4 in the specific activity for the ORR and a lower production of hydrogen peroxide in pure perchloric acid solution. The maximum activity of the Pt-based catalysts was found with ca. 30 similar to 40 at. % Ni content in the alloys, which could originate from the favorable Pt-Pt interatomic distance. The ring-current measurements on all the catalysts showed similar behavior for hydrogen peroxide production. The enhanced electrocatalytic activity of as-prepared Pt-Ni alloy catalysts for the ORR is attributed to the high dispersion of the alloy catalysts, to their disordered structure, and to the favorable Pt-Pt mean interatomic distance caused by alloying.