A series of transition metal phosphates, Mg0.5+y (Fe y Ti1−y )2(PO4)3 (MFTP), modified from Mg0.5Ti2(PO4)3 (MTP) were prepared by sol–gel method and evaluated as cathode materials for rechargeable magnesium cells. The crystal structure of MFTP at 0≤y≤0.5 was identical to that of MTP, while their unit cell volumes were smaller than that of MTP due to introduced Mg2+ ions. Electrochemical magnesium insertion into MFTP was found possible as observed for MTP. However, the limiting extent of magnesium insertion into MFTP at 0.1≤y≤0.5 was small compared with MTP and remarkably dependent on the current density during discharge, suggesting that the insertion limit is determined not by the number of available sites for Mg2+ or electrons but by the mobility of Mg2+ in the host varying with the unit cell volume.