Microscopic transition states and activation energies for formation reactions of divacancy-oxygen complex (V2O) are investigated by first-principles calculations. The calculations reveal that divacancy-oxygen complex is formed from divacancy or the vacancy-oxygen pair by reactions as V-2 + O-i -> V2O and VO + V -> V2O. The detailed analysis of calculation results clarified that the reaction of VO + V -> V2O occurs at room temperature, and the reaction of V-2 + O-i -> V2O occurs at 230-300 degrees C. The dissociation mechanisms of divacancy-oxygen complex are also discussed. (c) 2008 The Electrochemical Society.