MnP nanoparticles (MnPs) with a particle size of <30 nm were prepared by the reaction of dimanganese decacarbonyl and trioctyl phosphine (TOP) at 380 degrees C under Ar atmosphere. Lithium reactionmechanism of the MnPs was investigated by using electrochemical cycling, ex situ X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM). When lithium ion was first inserted into the MnP, it started to form LiMnP, LiP and LiP7 phases. During lithium dealloying (charging to 2 V), LiMnP phase was turned into the LiP and LiP5 phases. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.081205jes] All rights reserved.