AbstractThe neutral organophosphorus derivatives: phosphines, phosphine oxides, phosphinates, phosphonates, and phosphates are briefly reviewed. All form metal complexes and all except perhaps the first have been studied in some detail in connection with solvent extraction.The physical and chemical properties of these compounds are considered as is the mechanism of extraction for acids and metal salts. Most extractions are straightforward consisting of hydrogen bonding for acids and solvation for neutral salts. The strength of these complexes can often be correlated with infrared or nuclear magnetic resonance (NMR) shifts.Metals display a wide range of extractibility depending on the reagent (solvent and diluent), the aqueous anion, the water activity, and the acidity. Selective extraction is often found for nitrate, per chlorate, perhaloacetate, and some chelating acid salts.Because of the relative simplicity of the solvation complexes and the variability in the solvent strength of the different phosphorus esters, several attempts at thermodynamic studies of the extraction process have been made. Rigorous treatment of most systems will be seen to be difficult and the results have only been moderately satisfactory. Some recent work is discussed.For large scale uranium-plutonium processing, tributyl phosphate is firmly entrenched. Several alternatives have been considered including both alkyl and phenyl phosphonates. For small-scale applications and for analytical separations the more expensive phosphinates and phosphine oxides may be useful.