Direct Methanol Fuel Cells (DMFC) employing polymer Electrolyte Membrane (PEM) electrolytes have recently gained considerable interest because they are attractive candidates for vehicular applications. A. manifold of potential sources of performance losses exists for these systems, a very serious one being methanal crossover across the membrane electrolyte. This work is focused on the dependence of crossover on reaction conditions, such as temperature and methanal concentration. Anode performances are also evaluated. Results are given far the establishment of single electrode potentials during cell heat-up and of single electrode performances during operation under various conditions. Additionally, a cyclic voltammetry technique is presented, which allows the evaluation of methanal crossover in a fuel cell under operating conditions.