Dense La0.6Sr0.4CoO3-delta film electrodes were deposited by pulsed laser deposition (PLD) on Ce0.9Gd0.1O1.95 electrolytes. The grain size of one film was 300-500 nm, and the other was 30-50 nm. DC polarization and AC impedance measurements were performed at 873 K-1073 K in O-2-gas mixtures. From investigations of the electrochemical capacitances, the rate determining process for both electrodes were confirmed to be the surface reaction. The analyses in the electrochemical resistance revealed that the oxygen adsorption/desorption rate was faster on the electrode with smaller grain size. DC responses agreed with AC results, so the current density on the nano-grain electrode was larger by half an order than those of the sub-micron-grain electrode. Under a dilute oxygen atmosphere, the rate determining step transferred from a surface reaction to a gas phase diffusion. (c) 2006 Elsevier B.V. All rights reserved.