A direct titrimetric method for the determination of trivalent uranium in uranium trifluoride and mixtures of fused fluoride salts was developed. The method is based on the stoichiometric oxidation of trivalent uranium to the tetravalent oxidation state with an acidic solution of methylene blue. The sample containing trivalent uranium is dissolved at room temperature in an excess of standard methylene blue solution in a carbon dioxide atmosphere; the excess oxidant is variation of the method is 1.5 per cent for 5 mg quantities of trivalent uranium. The method was applied to various mixtures of fluoride salts containing both trivalent and tetravalent uranium.