Pure monoclinic Li3M2(PO4)3 (M: Fe, V) powders (<1μm in diameter) were obtained by an original route that involved initial homogenization of precursors in aqueous solution followed by slow evaporation and annealing under controlled atmosphere at moderate temperatures. The crystal structure of Li3V2(PO4)3 was determined for the first time through Rietveld refinements of neutron diffraction data. As for Li3Fe2(PO4)3, Li is distributed within three crystallographic sites, fully occupied at room temperature. The values of the temperature factors on Li(2) and Li(3) sites (five-fold coordination) were found significantly higher than that of Li(1) (four-fold coordination). Li3V2(PO4)3 shows four reversible redox phenomena upon insertion of two Li+ (V3+/V2+ couple), at 1.98, 1.88, 1.73 and 1.70V vs. Li. By comparison, Li3Fe2(PO4)3 shows two reversible redox phenomena upon insertion of two Li+ (Fe3+/Fe2+ couple), at 2.88 and 2.73V vs. Li. Experimental capacities close to the theoretical ones were obtained after optimal composite electrode preparation through ball-milling. In situ X-ray diffraction showed very minor changes from Li3M2(PO4)3 to Li5M2(PO4)3. Additionally, Li is extracted from Li3V2(PO4)3 towards V2(PO4)3 (V4+/V3+ and V5+/V4+ couples) through four redox phenomena at 3.59, 3.67, 4.06 and 4.35V vs. Li. Despite all these phase transitions, the [M2(PO4)3] framework is remarkably stable on cycling, particularly for M: Fe, while partial vanadium dissolution into the electrolyte occurs either on deep reduction to 1.5V or deep oxidation to 4.6V vs. Li.