This paper presents a combined computational and experimental study of the structural and electrochemical properties of monoclinic Li x M2(PO4)3 (with a focus on M=V). The voltage curve for x=0–3 Li is measured experimentally and calculated. Features of the voltage curve are understood as emerging from site energetics, Li ordering, and redox couples. These features are found to be largely independent of alloying and a simple additive model is proposed to analyze the voltage curve for different cation substitutions. The model is shown to be very useful for understanding experimental results for a number of substituted compounds.