An error analysis for real surface area (A(m)) measurements of Pt3Co/C catalysts is presented. The real surface area was measured by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), desorption of underpotential deposited hydrogen (H-upd) and CO-stripping voltammetry. All measurements were carried out after an electrochemical activation by cyclic voltammetry in perchloric acid. The real surface area decreases in the sequence A(m)(XRD) similar to A(m)(HRTEM) > A(m)(CO-stripping) A(m)(H-upd). HRTEM provides the highest reliability of the investigated methods. The electrochemical methods are questionable for Pt3Co/C catalysts. They yield surface areas which are 23-30% lower than determined by HRTEM. The surface coverage of adsorbed H and CO is apparently lower on Pt3Co compared to Pt nanoparticles. Cyclic voltammetry of Pt3Co/C in alkaline electrolyte suggests that the surface of acid leached Pt3Co nanoparticles consists of pure Pt. (C) 2010 Elsevier B.V. All rights reserved.