Nonstoichiometry is reported in the LiMnPO4/MnPO4 system. As lithium is removed from crystalline LiMnPO4 by chemical or electrochemical methods, the resulting two-phase mixture consists of stoichiometric LiMnPO4 and a delithiated phase, LiyMnPO4, whose lattice parameters depend on the global extent of delithiation and on the crystalline domain size of the delithiated phase. This behavior is reproduced during electrochemical insertion of lithium. Again, no evidence for nonstoichiometry was found in the vicinity of LiMnPO4. Attempts to create single-phase solid solutions by heating mixtures of the two phases failed due to the thermal instability of LiyMnPO4.