Oxygen reduction processes at a yttria stabilized zirconia\doped lanthanum manganite (LSM\YSZ) interface are investigated by ac impedance spectroscopy. Three semi-arcs are clearly observed at an oxygen partial pressure (p(O2)) of 0.001 atm. The constant-phase element of frequency arc and the p(O2)-dependence of 1/R (R: resistance) are correlated to interpret the reaction processes associated with the frequency arcs. It is found that at least five elementary steps are involved in oxygen reduction at the LSM\YSZ interface. Three of these steps are identified as the rate-determining steps, namely: (i) gas diffusion through the porous LSM electrode from the bulk to the reaction sites; (ii) surface diffusion of oxygen intermediate species along the LSM surface; (iii) incorporation of oxygen ions from the three phase boundary into the YSZ electrolyte lattice. The p(O2)-dependent exchange current density of the rate-determining step predicted by model is consistent with the experimental data, which further confirms the validity of the proposed oxygen reduction processes at the LSM\YSZ interface. (C) 2003 Elsevier Science B.V. All rights reserved.