A series of transition metal phosphates, Mg0.5+y (Fe y Ti1−y )2(PO4)3 (MFTP) and Mg0.5+y (Cr y Ti1−y )2(PO4)3 (MCTP), modified from Mg0.5Ti2(PO4)3 (MTP) were prepared by sol–gel method and evaluated as a possible cathode material for magnesium cells. The lattice structures of MFTP and MCTP at 0≤y≤0.5 were identical with that of MTP, while their unit cell volumes were smaller than that of MTP due to the increased amount of Mg2+ ions. Electrochemical magnesium insertion into MFTP and MCTP was found to be 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 discharge current density during discharge. These facts suggested that the insertion limit is determined not by the number of available sites for Mg2+ or electrons but by the mobility of Mg2+ in these host materials correlating with decrease in the unit cell volume.