Oxygen nonstoichiometry and oxide ion diffusion constant of La0.6Sr0.4Co0.8Fe0.2O3 - (delta) were gravimetrically studied with a CAHN1000 thermal balance, at temperatures 973-1073 K, with PO2 between 1 and 10(5) Pa. The weight relaxation was mainly determined by bulk diffusion, but also influenced by surface exchange reaction. Diffusion coefficients were obtained by fitting the experimental data to the solution of the diffusion equation. For log (Po-2/P-0)> - 2 (P-0= 101325 Pa), it was found that the oxygen vacancy diffusion constant is almost a constant, D-v=1.0 X 10(-5), for log (P-O2/P-0) < -2; however, a rapid decrease was observed. The surface reaction rate constant, k(chem) is two orders of magnitude higher than the chemical diffusion constant. It depends on oxygen partial pressure approximately in the way, k(chem) alpha P-O2(0.55) (C) 2003 Elsevier Science B.V. All rights reserved.