Conductivity relaxation experiments were conducted on La0.6Sr0.4CoO3-delta (LSC40) over a temperature range from 600 to 800 degrees C and oxygen partial pressure, p(O2), switch from 0.21 to 0.1 atm in O-2-N-2 atmospheres. Two types of samples were used: One was a bar-shaped dense sample of similar to 394 mu m thickness with polished surfaces. The other was an identical sample but with similar to 5 mu m porous surface layers of LSC40, which were applied to enhance kinetics of the surface exchange process. It was observed that the kinetics of conductivity relaxation in the sample with porous layers was much faster than in the sample without porous layers, and data fitting on the sample with porous surface layers was only sensitive to the chemical diffusion coefficient, (D) over tilde, but not to the surface exchange process. Using (D) over tilde estimated from the sample with porous layers, the chemical surface exchange coefficient, k(chem), was determined from conductivity relaxation data on the sample without porous layers. These (D) over tilde and k(chem) are presumed to be accurate values of the parameters for the material under the measurement conditions (temperature and p(O2)). It was observed that fitting the conductivity relaxation data on the sample without porous layers by simultaneously varying k(chem) and (D) over tilde, however, led to significant uncertainties in both of the estimated parameters. In general, fitting the data obtained on the polished sample to k(chem) and (D) over tilde simultaneously led to an overestimation of k(chem) and underestimation of (D) over tilde. The present work also shows that the measured k(chem) is expected to be substantially affected by the surface preparation procedure used. (c) 2006 The Electrochemical Society.