The effect of cathode structure on the performance of planar free-breathing fuel cell is studied. Three different types of gas diffusion backings and current collector plates on the cathode were used. The gas diffusion backings used were thick carbon sheet, titanium sinter, and carbon paper. Difference between current collector plates was the size of the openings between current collecting ribs. Results showed that with thicker gas diffusion backings the structure of the current collector had only some effect on the cell performance but with thin carbon paper the effect was significant. Very high power densities for free-breathing fuel cell were achieved with thin gas diffusion backing with the record of approximately 360 mWcm(-2). However, the cell was also vulnerable for flooding as there was liquid water observed on the cathode surface. Despite the liquid water saturation it seems that this kind of cell design may be suitable especially for portable applications. (C) 2004 Elsevier B.V. All rights reserved.