Tin hydrogen phosphate (SnHPO4) has been tested as an electrode material in Li cells. It exhibits a high capacity for the first discharge and a capacity of ca. 200 mAh g(Sn)(-1) after 25 cycles at 20degreesC. The Li-to-network interaction during the first discharge has been followed using X-ray diffraction and Li-7, P-31 magic-angle spinning nuclear magnetic resonance. The results obtained allow us to propose a Li-reaction mechanism for this phase, based on the formation of Sn and Li 3 PO 4 in the first stage followed by reversible Sn/Li alloying.