This paper deals with the reaction dynamics of phase transition for the electrochemical lithium insertion into Cu2Sb intermetallic compound. Electrochemical lithium insertion/removal, alternating current (ac) impedance technique, and electrochemical calorimetric measurement have been applied in this study. The electrochemical charge-discharge test indicated a large irreversible capacity at the onset of first charging (Li insertion), where a flat voltage plateau appeared around 0.7 V. In other instances, following cycles showed good reversibility. In addition, the voltage profiles of > 0.7 V region after the second cycle showed higher than the one at the first cycle charging and clear solid solution manner. Thus, the difference of the electrochemical Li insertion mechanism between the first and following cycles caused irreversibility accompanying large electrochemical polarization. To explain the above phenomena, we assumed that the nucleation was rate determined at the onset of phase transition from tetragonal Cu2Sb to cubic Li1+xCuSb. After the phase transition was completed, following cycles did not need to expend the formation of different phase nuclear, showing a good reversibility reaction. The reaction mechanism suggested above was supported by ac impedance and electrochemical calorimetric measurements. (c) 2007 The Electrochemical Society.