The electrocatalytic activity of Pt/Co bimetallic nanoparticles towards formic acid (0.2 M HCOONa/0.28 M HClO4) oxidation was explored by cyclic voltammetry of the nanoparticles supported on highly oriented pyrolytic graphite (HOPG). The particles were prepared by electrodeposition from a bath of mixed concentrations of Co2+ and PtCl62- ions and characterized by tapping mode atomic force microscopy (TMAFM). The compositions of the nanoparticles were determined as close as possible to individual particles by energy dispersive X-ray analysis (EDX). Nanoparticles of different atomic ratios of Pt/Co were prepared and the peak currents in cyclic voltammetry of formic acid oxidation were measured. The activity of the bimetallic particles was found to be maximum when the atom ratio Pt:Co is between 1 : 1.1 and 1 : 3.5. The maximum activity is about one order of magnitude higher than that of pure Pt nanoparticles. Comparison of the currents to those from microelectrodes and wire electrodes shows no mass-transfer limitations in nanoparticles with diameters less than 100 nm. The bimetallic particles exhibit interesting kinetics and Co appears to significantly enhance the electroxidation of formic acid. The advantage of using nanoparticles for mass-transfer free kinetics studies is also demonstrated.