The distribution of thori~un and a nunlber of lanthanides between nitric acid and solutions of dibutyl butyl phosphonate in odorless kerosene has been examined as a function of the aqueous nitric acid conce~ltration. Experiments \Irere conducted a t trace metal concentration using radioisotopes. Separation factors (denoted by S a n d defined as the ratio of the distribution coefficients, K , for two metal species) have been measured for some lanthanide-lantha~lide couples and also for some thori~1111-lanthanidc couples. Results indicate that separation factors bcttveen successive lanthanides (gixen by S = Kz+l/Kz) a t the lower end of the rareearth series are superior to those obtained with either tributyl phosphate (TBI') (D. Scargill et al.