Solid solutions of LiFe1-yMyPO4/C (M = Co, Ni) within the whole substitution range (0 <= y <= 1) are systematically investigated as safe high-voltage positive electrode materials for Li-ion battery. Especially the similarities and differences between the Co- and Ni-substitution schemes are highlighted. With increasing substitution level y, the orthorhombic unit cell shrinks and the Li+-ion diffusion channel area decreases, more rapidly for the smaller-sized Ni substituent. While the Co2+/Co3+ redox couple shows reversible electrochemical activity, only an initial, partial delithiation is achieved for the Ni2+/Ni3+ couple at all y. However, both substitution schemes similarly affect the Fe2+/Fe3+ redox couple by increasing its potential and enhancing the kinetics. Particularly, a mutual influence of Fe and Co/Ni on the delithation/lithiation characteristics is proposed: ex situ XRD analysis shows signs of a phase-composition change in the Fe2+/Fe3+ region for higher y, and correspondingly, a changed Co2+/Co3+ redox mechanism for lower y is indicated by cyclic voltammetry. We suggest that this behavior is related to a substitution-induced decrease in the volume change between the lithiated and delithiated phases. For the Co-substitution scheme, a composition around y approximate to 0.5 seems optimal for combining a high energy density and a kinetically beneficial, diffusional single-phase delithiation/lithiation mechanism. (C) The Author(s) 2015. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. All rights reserved.