O-2 reduction reactions were investigated on (La,Sr)MnO3 (LSM) and (La,Sr)(Co,Fe)O-3 (LSCF) electrodes at temperatures from 700 to 900 degreesC in air with and without the presence of gaseous Cr species. Gaseous Cr species were introduced to the system by using chromia-forming alloy interconnect in contact with the electrode. For 02 reduction on the LSM electrode, three reaction steps have been identified: surface dissociative adsorption and diffusion, charge transfer and oxygen ion migration into the zirconia electrolyte phase. The reaction is controlled by the dissociative adsorption and diffusion at LSM electrode surface at low temperatures and the oxygen ion migration/diffusion into zirconia electrolyte at high temperatures, By comparing the electrode behavior of LSM and LSCF in the absence and presence of chromia-forming alloy under identical experimental conditions, reaction steps on LSCF electrodes have also been identified. The results indicate that both surface and bulk diffusion processes play important roles in the overall reaction kinetics for the O-2 reduction on LSCF electrodes, (0 2002 Elsevier Science B.V. All rights reserved.