Extraction of tri-, tetra- and hexavalent metal ions by N, N-N',N'— octabutylimidodiphosphotetramide has been investigated. Imidodiphosphoryl extractants show promise for treatment of nuclear waste but the optimum extractant structure must still be determined. Extraction of rare earths was investigated using two—phase potentiometnc titrations as the more conventional slope analysis techniques could not be applied. The data thus obtained were analysed by the ESTA library of programs which was designed for modelling solution equilibria in aqueous phases. Results obtained showed that ESTA may be used for modelling two—phase systems providing corrections are made. Models were obtained for the extraction of Nd(III), Eu(III), Y(III) and Yb(III). Extraction equilibrium constants could be calculated as well as distribution ratios showing that imidodiphosphoryl extractants are promising as rare earth extractants. Extraction mechanisms