Cathode catalyst layers were prepared and characterized as part of membrane electrode assemblies (MEA) and catalyst coated membranes (CCM) on the basis of carbon supported methanol tolerant RuSe (x) catalysts. Preparation parameters varied were: catalyst loading (0.5-2 mg RuSe (x) cm(-2)s), PTFE content (0, 6, 18 wt.%), carbon support (Vulcan XC 72 or BP2000), and fraction of RuSe (x) in the carbon supported catalysts (20, 44, 47 wt.%). The MEAs and cathode catalyst layers were electrochemically characterized under Direct Methanol Fuel Cell (DMFC) operating conditions by recording polarization curves, galvanostatic measurements, and impedance spectra. The morphology of the catalyst layers was investigated by means of confocal laser scan microscopy (CLSM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) measurements. MEAs with Ru(44.0 wt.%)Se(2.8 wt.%)/VulcanXC72 cathode catalyst achieved the highest performance of all RuSe (x) catalysts investigated, i.e. similar to 40 mW cm(-2)s at 80 degrees C under ambient pressure and lambda(MeOH) = lambda(air) = 4. This is 40% of the value obtained with commercial platinum cathode catalyst under the same operating conditions. The RuSe (x) catalysts investigated are stable over a period of more than 1,000 h. This was confirmed by TEM and XRD measurements, where no increase in mean RuSe (x) particle size (similar to 5 nm) after fuel cell operation was found. Enhancement of specific catalyst activity, mass transport, and active surface offer potential for a further improvement of RuSe (x) catalyst layers.