The oxygen reduction reaction can only proceed at locations where gas, electronic conductor, and an oxygen ion conductor meet. Although the extension of the reaction zone beyond the traditional so-called triple-phase-boundary (TPB) is widely accepted for a mixed ionically and electronically conductive cathode, work in this area has yet to reach a consensus on how far the reaction zone can be extended. In this study, anode-supported fuel cells with a variety of LSCF cathode thicknesses were fabricated and tested in two cathode environments, flowing oxygen and flowing air. In flowing oxygen, the cell performance increased with LSCF cathode thickness over the entire range investigated (from 5 to 33 mu m) because of the increased number of reaction sites. In flowing air, the cell performance also increased with the LSCF cathode thickness from 5 to 13 mu m, but then remained almost constant with further increase in cathode thickness due to depletion of oxygen beyond a certain thickness. Published by Elsevier B.V.