A series of Pt/C electrocatalysts with average Pt particle size of 1.8, 2.3, 3.4, 3.8, 4.7, and 5.8 nm are synthesized by reducing platinum(II) acetylacetonate with 1,2-hexadecanediol in the presence of long-chain carboxylic acid and alkylamine stabilizing agents. The prepared materials are characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and energy dispersive X-ray (EDX) spectrometry. The face-centered cubic structure of Pt in the materials is evident from XRD. Good spatial distribution of spherical shaped Pt nanoparticles is seen from the HRTEM images. The presence of Pt and C is observed from the EDX analysis. Linear sweep voltammetry (LSV), rotating disk electrode (RDE), and single-cell proton exchange membrane fuel cell (PEMFC) measurements are conducted to evaluate the electrocatalytic activity of Pt/C electrocatalysts. Electrochemical measurements indicate the good hydrogen oxidation and oxygen reduction activity for 1.8 and 3.4 nm size Pt particles, respectively. Kinetic analysis by RDE voltammetry reveals that the number of electrons transferred in hydrogen oxidation (HOR) and oxygen reduction (ORR) processes on 1.8 and 3.4 nm size Pt nanoparticles is 2 and 4, respectively. The high performance Pt/C anode and cathode are then used to fabricate a membrane-electrode assembly (MEA) and tested in a PEMFC. A maximum power density of 625, 760, and 1030 mW/cm2 is observed at 333, 343, and 353 K, respectively. © 2010 American Chemical Society.