Catalyst materials in inks, in half-MEAs and in a polymer electrolyte membrane (PEM) fuel cell were characterized by cyclic voltammetry. Lab.-made Pt inks were modified with Ru, Os and Ru+Ir and the MeOH oxidn. activity of the created surfaces was tested. A novel electrode, which enables the study of the O redn. reaction on the cathode was introduced. With this measuring device it was possible to vary the O content on the ink side and the p 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 are linearly dependent on the Pt mass d. in the catalyst layers. Approx. 30% of the Pt loading in the MEA was electroactive.