A series of transition metal phosphates, Mg0.5+y(FeyTi1-y)(3) (PO4)(3) (MFTP), modified from Mg0.5Ti2(PO4)(3) (MTP) were prepared by solgel method and evaluated as cathode materials for rechargeable magesium cells. The crystal structure of MFTP at 0 less than or equal to y less than or equal to 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 less than or equal to y less than or equal to 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.