The effect of cathode composition, processing and structure on the performance of medium-temperature (600-800 degrees C) solid oxide fuel cells (SOFCs) is described. The cathodes and physical supports for the SOFCs were two-phase mixtures of (La1-xSrx)(1-y)MnO3 (LSM) and Yttria-stabilized Zirconia (YSZ), the electrolytes were < 10 mu m thick YSZ, and the anodes were Ni-YSZ with Y-doped CeO2 interfacial layers. It was found that the cathode overpotential was the primary factor limiting cell power densities during operation with air as the oxidant and humidified hydrogen as the fuel. Increasing the YSZ volume fraction in LSM-YSZ cathodes from 0 to 60% reduced the low-current area-specific resistance of the cells (in air and humidified hydrogen) from similar to 3.3 to 0.7 Omega cm(2). The use of LSM with y = 0.1 suppressed the formation of zirconate phases during cathode sintering. Optimal cathode porosity was approximate to 40%. Decreasing the cathode porosity below approximate to 30% resulted in a mass transport limitation at high current densities due to the small pore size (< 0.5 mu m) and large cathode thickness (approximate to 1 mm). The maximum power densities measured in humidified H-2 and air ranged from similar to 110 mW cm(-2) at 600 to 470 mW cm(-2) at 800 degrees C.