A quantitative study of the extraction of tetravalent uranium into benzene by the action of the organic chelating agent thiophenoyl trifluoracetone is described. Data are presented to show that the uranous ion exists mainly in the quadruply charged form in solutions of perchloric acid. It is extracted into benzene as a neutral molecule containing four chelate groups per metal atom. At least two complex species, U(SO4)++and U(SO4)2are present in sulphuric acid solutions. Values of the association constants for the formation of these complex species are given.