The phase formation and solid solid solution system of Lithium Iron (II) Phosphate and Lithium Manganese (II) Phosphate using solid state reaction have been studied. The observed phases were identified by X-ray powder diffraction and Electrochemical measurements. The synthesised  phases show X-ray diffraction spectra typical of orthorhombic symmetry, space group Pmnb and are closely related to that of the olivine system. A relation between the amount of  δ  in the system Li_x (Fe_1-δ Mn_δ) PO₄ and the diffraction lines 2θ (hk1) has been found. The  electrochemical and chemical analyses of the charge - discharge reaction mechanism of the orthorhombic type Li_x (Fe_1-δ Mn_δ) PO₄ , 0< x<1, showed that this compounds can be used as cathode material for lithium rechargeable batteries.