A combustion synthesis technique was used to prepare nanoparticulate LiMgxMn1−xPO4 (x = 0, 0.1, 0.2)/carbon composites. Powders consisted of carbon-coated particles about 30 nm in diameter, which were partly agglomerated into larger secondary particles. The utilization of the active materials in lithium cells depended most strongly on the post-treatment and the Mg content and was not influenced by the amount of carbon. Best results were achieved with a hydrothermally treated LiMg0.2Mn0.8PO4/C composite, which exhibited close to 50% utilization of the theoretical capacity at a C/2 discharge rate.