The impedance responses and current-overpotential characteristics were studied for oxygen reducing cathodes, stoichiometric La1-xSrxMnO3 (x = 0.16-0.20) in solid oxide fuel cells, under varying conditions of temperature (700-900 degrees C) and oxygen partial pressure (1-10(-4) atm) for two distinctly different electrode morphologies (porous and dense structure). An electrode densified at the interface with the yttria-stabilized zirconia solid electrolyte was more efficient in this study. The reaction at the finely porous LSM proceeds via the triple phase boundary and ineffectively uses the mixed conductivity property of the material. Reaction mechanisms for both cases (porous and dense structure) are discussed and compared with the literature. A new, two-layered cathode structure is proposed.