The complex structural transformations of Li0.5Ni0.25TiOPO4 during electrochemical lithiation have been examined by in situ X-ray diffraction. During the first lithiation two structural changes take place: first a transition to a second monoclinic phase (a = 9.085(4), b = 8.414(5), c = 6.886(5), beta = 99.85(4)) and secondly a transition to a third phase with limited long-range order. The third phase is held together by a network of corner sharing Ti-O octahedra and phosphate ions with disordered Ni-Li channels. During delithiation the third phase is partially transformed back to a slightly disordered original phase, Li0.5Ni0.25TiOPO4 without formation of the second intermediate phase. These phase transitions correspond well to the different voltage plateaus that this material shows during electrochemical cycling. (C) 2011 Elsevier B.V. All rights reserved.