A technique using conductivity relaxation on porous bodies was used to determine the oxygen surface exchange coefficient, k(chem), of mixed conducting La0.6Sr0.4CoO3-delta (LSC40). The results from the porous bodies were compared with values of kchem obtained on dense samples of LSC40 using the same technique. Conductivity relaxation experiments were conducted over a temperature range from 600 to 800 degrees C for small step changes in partial pressure of oxygen, po(2), both decreasing and increasing, from 0.02 to 0.21 atm. The kchem obtained on a porous body at a given temperature was found to increase in the range from 10(-6) to 10(-5) cm/s over the po(2) range from 0.02 to 0.21 atm, with leveling off occurring at the higher po(2) values close to 0.21 atm. Over the temperature range investigated, kchem was found to increase with decreasing temperature, with the dependence described by an enthalpy of -23 kJ/mol. This contrasted sharply with the results obtained on dense samples in which k(chem) was found to increase with increasing temperature, with the corresponding activation energy of 220 kJ/mol. This discrepancy was rationalized on the premise that in dense samples of the thickness used, transport may have been predominantly controlled by chemical diffusion, which made accurate determination of kchem difficult. The increasing k(chem) with decreasing temperature in porous samples may indicate that the adsorption reaction is at equilibrium and the temperature dependence reflects the temperature dependence of surface coverage. (c) 2005 The Electrochemical Society. All rights reserved.