The relatively high redox potential in the olivine-type LiMPO4 (M =Mn, Fe, Co, Ni) materials has largely been explained by the M–O–P inductive effect which increases the ionic character of transition metal atoms. Here, we identify the additional perturbative effect that slightly but systematically shifts the redox potential. The substitution of iron by manganese in the olivine LiMPO4 framework raises both of the Fe3+/Fe2+ and Mn3+/Mn2+ redox potentials by ∼0.1V. The overall volume expansion upon Mn substitution in the whole Li x (Mn y Fe1−y )PO4 system possibly increases the average metal-oxide bond length and hence the ionicity of each transition metal. The voltage shift in a single cell is small but should be significant in a larger-scale battery where there exist a large number of series connections. The kinetic shift for each of the Fe3+/Fe2+ and Mn3+/Mn2+ redox reactions is also investigated.