The room temperature (x, y) two-dimensional phase diagram of the olivine-type solid-solution, Li-x(MnyFe1-y)PO4 (0 less than or equal to x, y less than or equal to 1, orthorhombic, D-2h(16):Pmnb), is determined. The x-dependent changes in the unit cell dimensions at various fixed Mn contents y are analyzed in detail. The manganese substitution for iron in the octahedral 4c sites induces 1, the two-phase Mn3+/Mn2+ redox region with a potential of 4.1 V vs. Li/Li+1; 2, a partial conversion of the form of the Fe3+/Fe2+ redox reaction (3.4 V vs. Li/Li+) from two-phase to single-phase; and 3, phase instability at the composition region close to the point (x, y) = (0, 1) caused by the Jahn-Teller effect of Mn3+. The conversion, 2, is complete at around y = 0.6, The phase instability, 3, makes the Mn-rich phase (y > 0.8) unsuitable for battery applications. The local lattice deformation around Mn3+ is severe enough to induce significant selective damping in the extended X-ray absorption fine structure for Mn3+.