The oxygen surface exchange coefficient k(chem) of a La0.5Sr0.5CoO3-delta (LSCO) thin film has been determined from electrical conductivity relaxation measurements. The LSCO thin films were deposited on LaAlO3 (LAO) single crystal substrates by pulsed laser deposition (PLD). The electrical conductivity relaxation behavior of the film was measured at high temperature on switching the oxygen partial pressure between 0.01, 0.05, 0.10, 0.30, 0.50 and 1.00 atm. The k(chem) values were obtained by fitting the conductivity relaxation curves using a surface-limited kinetics model. The results show that k(chem) increases with temperature and with the oxygen partial pressure after the switch, but is not sensitive to the initial partial pressure. After prolonged heating at 900 degreesC, k(chem) increased substantially. The increase is associated with a change in the thin film surface morphology on prolonged heating.