Dimethyl ether (DME) combines high energy density with easy handling and low toxicity and is therefore an attractive fuel. The absence of carbon-carbon bonds allows for electro-oxidation with good kinetics and it is therefore particularly interesting for use in fuel cells. This work presents the first durability studies of vapor-fed direct dimethyl ether fuel cells with phosphoric acid doped polybenzimidazole membranes as electrolytes. Fuel cells are operated in direct DME mode at 160 and 200 degrees C and the cell voltage at a constant current load of 100 mA cm(-2) is recorded over more than 200 h. Regular electrochemical impedance spectroscopy and polarization data are used as diagnostic measures to monitor the cell characteristics. It is shown that the cell performance deteriorates severely within 200 h of operation at 160 or 200 degrees C. The degradation is connected to different modes that ultimately result in both increasing polarization resistance and increasing area specific resistance, which may be connected to the chemical incompatibility between the fuel and the electrolyte.