The method of the invention for preparing a solution of a rare-earth organophosphate in an organic solvent comprises reacting, in this solvent, a rare-earth compound chosen from rare-earth oxides, hydroxides, carbonates and hydroxycarbonates with an organophosphorus acid and in the presence of a promoter chosen from water, nitric, hydrochloric, acetic, formic and propionic acids and the rare-earth salts of these acids. The use of a promoter makes it possible to simplify the method, to reduce the reaction time and to obtain a solution for which the residual solids content may be low.