The electrochemical oxidation of CO on carbon-supported electrocatalysts was studied with different amounts of platinum and molybdenum as active phases. These materials were synthesized by the metal carbonyl thermolysis method. Molybdenum hexacarbonyl and large or small polymeric chains of Pt carbonyl complex were used as metal precursors. The catalysts were characterized by XRD, HRTEM and SEM-EDX. The platinum active area was determined by voltammetric CO stripping. The electrochemical methods revealed an enhanced performance of the platinum-molybdenum materials for the CO oxidation, in comparison with those formulated only with Pt. The best results were obtained for low molybdenum loadings. The lowest onset oxidation potential was obtained with a 4:1 relationship in the Pt:Mo content, which indicates the highest activity for this catalyst for CO oxidation. The presence of molybdenum caused a decrease in the catalytic active area of the materials synthesized with large platinum particles, whereas the opposite was observed with the catalysts synthesized with small Pt particles, where the addition of Mo results in higher active areas than the Pt catalyst. (C) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.