The structural evolution of multi-component olivines upon delithiation and lithiation was investigated by ex-situ X-ray diffraction. We found that one-phase de/lithiation occurs for multi-component olivines for a fairly large compositional range of transition metal species given that two-phase de/lithiation is generally accepted for olivine cathodes. We discuss the driving force that determines the phase behavior (two-phase reaction vs. one-phase reaction) of olivines in terms of enthalpy of mixing presumably dominated from the electrostatic interaction among cations and the elastic strain stored in the crystal due to the volume change of redox active element. We believe that this fundamental study on the phase behavior of olivines will provide insight into the intrinsic electrochemical properties of olivine electrodes for rechargeable lithium batteries. (C) 2013 The Electrochemical Society. [DOI: 10.114912.036303jes] All rights reserved.