Carbon-supported Pt-Co alloy nanoparticle catalysts of nominal atomic composition but with different alloying extents were prepared via a modified Watanabe process by employing microwave heating. Their structure was studied by X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) techniques. Transmission electron microscopy (TEM) images indicated that the in-house-prepared Pt-Co alloy nanoparticles (sample-1 and sample-2) were well dispersed on the surface of the carbon support with narrow particle size distribution which is consistent with XRD grain size values. The catalyst composition obtained from XAS was nearly the same as that of the nominal value (1:1). The alloying extent values of Pt calculated from XAS measurements for sample-2 is similar to that of the commercial E-tek Pt-Co/C. This observation demonstrates the practical viability of our preparation protocol for Pt-Co/C catalysts. A comparative study was made for the oxygen reduction reaction (ORR) using a thin-film rotating disk electrode method to evaluate the catalytic behavior of Pt-Co/C and Pt/C catalyst with similar metal loadings and particle sizes. As compared to the Pt/C catalyst, the bimetallic Pt-Co/C of sample-2 exhibited an enhancement factor of 3 in mass activity at 0.95 V toward ORR. The enhancement in activity of sample-2 is similar in magnitude to that of the commercial E-tek PtCo/C catalyst. It is found that that the samples possessing a high alloying extent of Pt in the cluster enhances the activity of the bimetallic Pt-Co/C toward ORR. This observation confirms that the alloying extent of Pt is an important parameter by which one can have control over the fine-tuning of the catalytic activities of bimetallic nanoclusters. This activity enhancement may originate from the favorable electronic effects of a well, mixed alloy underneath a thin "Pt-rich skin" structure of the Pt-Co bimetallic nanoparticles. This kind of thin "Pt-rich skin" is created by the dissolution of Co oxide on Pt-Co bimetallic nanoparticles while washing in acidic electrolyte before being subjected to ORR.