The invention relates to a process for the preparation of a carbon-treated complex oxide having a very low water content and to its use as cathode material.The carbon-treated complex oxide is composed of particles of a compound AMXO4 having an olivine structure which carry, on at least a portion of their surface, a film of carbon deposited by pyrolysis. A represents Li, alone or partially replaced by at most 10% as atoms of Na or K. M represents Fe(II), alone or partially replaced by at most 50% as atoms of one or more other metals chosen from Mn, Ni and Co, and/or by at most 10% as atoms of one or more aliovalent or isovalent metals other than Mn, Ni or Co, and/or by at most 5% as atoms of Fe(III). X4 represents PO4, alone or partially replaced by at most 10 mol % of SO4 and SiO4. Said material has a water content