From freezing-point and isopiestic studies, it has been found that the acidic organophosphorus extractants of the type (GO)2PO(OH) and GOG′PO(OH) (where G is aryl, alkyl, or a derivative thereof) are predominantly dimeric in nonpolar diluents, such as n-hexane, cyclohexane, benzene, and carbon tetrachloride. The dimers are very stable and appear to monomerize only in more polar solvents, such as acetone, chloroform, or ethanol. Infrared spectra of these acids have been made, and the positions of the phosphoryl absorption determined in terms of the electronegativity of the G group. It is concluded from these studies that for the acids (GO)2PO(OH) an ionic P+O− and a strong acid is the better extractant; while for the acids GOG′PO(OH) a phosphoryl group with high double bond character and a strong acid is a better extractant. For both acid types these criteria are provided whenever the G group is highly electronegative. Complexes of organophosphorus extractants with metallic nitrates have been studied by infrared methods and it has been concluded that the nitrate spectra is of lowered symmetry than is present in ionic nitrate spectra, indicating a higher degree of covalency in the metal to nitrate band. Examples of these complexes are RE(NO3)3·2TBP (where RE is a rare earth metal, TBP is tri n-butyl phosphate) and M(NO3)(DEHP)3 (where DEHP is bis-(2-ethylhexyl)phosphoric anion). Infrared investigations of the salts of the acidic organophosphorus extractants have indicated the appearance of 2-POO− absorptions (asymmetric and symmetric). The asymmetric absorption appears to vary (1200–1282 cm−1) with the nature of the G group (being at higher frequency where G is aryl), and with the ionic potential of the metal (being at higher frequency as the ionic potential increases). Recent ultraviolet and infrared studies of complexes of the type M/TTA/TBP have been made and certain structures for these compounds have been indicated.