The durability of solid oxide fuel cell (SOFC) composite cathodes of lanthanum strontium manganite and yttria stabilised zirconia was investigated. The cathodes were kept at constant, realistic operating conditions (-300 mA cm(-2) at 1000 degrees C in air) for up to 2000 h. After the 2000 h test the increase in electrode overvoltage exceeded 100% of the initial value. Nominally identical cathodes kept for 2000 h at 1000 degrees C in air without current load for comparison showed little or no degradation. Thus, the current load of -300 mA cm(-)2, rather than the operation temperature of 1000 degrees C, was responsible for the degradation. Structural analysis showed an increase in the porosity at the electrode interfaces, when the electrode had been polarised. No such structural changes were found for electrodes tested without current load. The degradation is primarily ascribed to pore formation in the electrode material induced by an electric field.