Catalyst materials in form of inks, as half-MEAs and inside a polymer electrolyte membrane unit cell were characterized by cyclic voltammetry. Laboratory made Pt inks were modified with Ru, Os and Ru + Ir and the methanol oxidation activity of the created surfaces was tested. A novel electrode, which enables the study of the oxygen reduction reaction on the cathode was introduced. With the measuring device it was possible to vary the oxygen content on the ink side and the proton activity of the electrolyte on the membrane side of a half-MEA. Catalyst layers based on Pt, Pt-Ru and Pt-Co inside a PEM-unit cell were also characterized in situ by cyclic voltammetry. The active surface areas were found to be linearly dependent on the Pt mass density in the catalyst layers. Approximately 30% of the Pt loading in the MEAs were electroactive. (C) 2003 Elsevier Science B.V. All rights reserved.