In this work, carbon-supported platinum nanoparticles were synthesized in the presence of varying concentrations of thiourea (TU). Because of the presence of the sulfur atom, thiourea adsorbs strongly on the forming Pt nuclei during the synthesis, affecting the rate and the extent of growth. High-intensity X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) were used to probe the effect on the crystallite size and particle size, respectively, revealing a decrease in both mean crystallite and the mean particle size for increasing concentrations of thiourea during the synthesis. An adsorption isotherm for the adsorption of thiourea on platinum nanoparticles is also discussed. Cyclic voltammetry was used to check the effect of TU on the particles as well as to obtain further knowledge on both adsorption and electrochemical oxidation behavior of thiourea on Pt surfaces. Thermal treatment effectively cleaned the electrode surface, thus providing an easy way to remove adsorbed species from the Pt surface. (c) 2007 Elsevier B.V. All rights reserved.