The linear polarization behavior of La1-xSrxCoO3-delta electrodes has been measured as a function of x, T, and P-o2 using a.c. impedance. These measurements indicate a reaction mechanism in which O-2 is reduced chemically at the porous mixed conductor surface, followed by diffusion of oxygen through the mixed conductor to the electrolyte. Analysis of the electrode kinetics using a recent model for this mechanism yields values for the oxygen vacancy diffusion coefficient (D-v) and surface-exchange rate constant (r(o)) as a function of x, T, and P-o2. These values agree well with published independent measurements of oxygen chemical diffusion and isotope surface exchange ((D) over tilde and k). The distance (delta) that the reaction extends beyond the electrode/electrolyte interface depends on both D-v and r(o), varying between 0.3 and 10 microns for a typical electrode surface area.