The distribution of thorium and a number 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 concentration. Experiments were conducted at trace metal concentration using radioisotopes. Separation factors (denoted bySand defined as the ratio of the distribution coefficients,K, for two metal species) have been measured for some lanthanide–lanthanide couples and also for some thorium–lanthanide couples. Results indicate that separation factors between successive lanthanides (given byS = KZ+1/KZ) at the lower end of the rare-earth series are superior to those obtained with either tributyl phosphate (TBP) (D. Scargilletal. J. Inorg. & Nuclear Chem. 4, 304 (1957)) or trioctyl phosphine oxide (TOPO) (J. M. Schmitt. Oak Ridge National Laboratory. Unpublished data), but asZincreases,SDBBP~STBP > STOPO. For thorium–lanthanide couples,S′DBBP > S′TBP. Measurements over a range of extractant concentrations indicate that the lanthanides are extracted as trisolvates.