The solubilities of mono- and di- n-butylphosphoric acid (H 2MBP and HDBP respectively) at 25°C in water, aqueous nitric acid, odourless kerosene and benzene have been measured using 32P-labelled acids. The net distribution coefficient ( D HDBP) for HDBP has been measured as a function of the concentration of HDBP for 1. (i) a number of organic solvents in contact with 1 M HNO 3, 2. (ii) chloroform in contact with 0·1 and 1 M perchloric acid, and 3. (iii) the kerosene-HNO 3 system from 0 to 15 M HNO 3 In TBP-kerosene-HNO 3 systems, D HDBP and D H 2MBP have been measured as functions of the concentrations of HDBP, H 2MBP, TBP, and HNO 3, and TBP has been shown to associate with both HDBP and H 2MBP. The data can be applied to the calculation of equilibrium distribution coefficients for HDBP and H 2MBP in TBP-HNO 3 systems over a range of [TBP] from 0·1–100% v/v in the diluent, of [HNO 3] from 0 to 15 M, and of [HDBP] and [H 2MBP] either separately or together in the range 10 −6 M-0·05 M. The solubility and distribution data for HDBP are qualitatively interpreted in terms of 1. (i) a series of equations for the dissociation of HDBP, the dimerization of HDBP in both aqueous and organic phases, the association of HDBP with TBP and with HNO 3, and the association of TBP with HNO 3, and 2. (ii) the distribution of the HDBP monomer and dimer, and the HDBP·HNO 3 species, between the phases. The dimerization constants of HDBP in various organic solvents, the distribution constants of the monomer and the dimer in aqueous-organic solvent systems, and the association constant between TBP and HDBP in kerosene have been calculated.