The mechanism by which LiFePO4 is transformed into isostructural FePO4 has been elucidated using electron microscopy on large, hydrothermally grown LiFePO4 crystals following chemical delithiation. Lithium is extracted at narrow, disordered transition zones on the ac crystal surface as the phase boundary progresses in the direction of the a-axis. The substantial lattice mismatch along a (ca. 5%) causes crack formation in the bc plane. Despite considerable disorder in the transition zone, the general structural arrangement is preserved, leading to good crystallinity in the newly created FePO4 domains. Implications for improved electrode performance are discussed.