The effects of water on oxygen surface exchange kinetics and its degradation of dense La(0.6)Sr(0.4)CoO(3-delta) (LSC) thin films on yttria-stabilized zirconia (YSZ) are investigated by impedance spectroscopy and Xray photoelectron spectroscopy (XPS). Two distinguishable effects of water on oxygen surface exchange are identified. While humidification of the oxidant accelerates the oxygen incorporation in the short-term range, the long-term stability of the cathode decreases substantially. XPS analysis indicates that the accelerated LSC degradation rate under wet conditions originated from an increased change of the surface cation composition. (C) 2011 Elsevier B.V. All rights reserved.