A detailed study of the topotactic reaction of lithium insertion was undergone in Mg0.5Ti2(PO4)(3). For this purpose, a large number of in-situ XRD patterns were recorded during the first cycle evidencing a solid solution system. The full pattern refinement revealed an anisotropic variation of the cell parameters while M-1 sites were filled, leading to constancy in the cell volume. Otherwise, cell parameters monotonically increased when the M-2 sites are occupied by the incoming lithium ions. The high reversibility of the deinsertion process led to the recovery of the initial cell parameters at the end of the charge. The lithium cell delivered 119 mAh/g after the first discharge and values as high as 110 mAh/g were kept after 15 cycles at C/20. Moreover, the capability of inserting sodium ions was also demonstrated. Although a loss of capacity was inevitable, 97 mAh/g were recorded for the first discharge. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.060210jes] All rights reserved.