AbstractIn general, the mono-acidic phosphates and phosphonates are dimeric and the di-acidic phosphates and phosphonic acids are polymeric in the diluents commonly employed in metal extraction studies. Therefore, they may be symbolized, respectively, as (HY)2and (H2Y)x.The extraction of tracer-level M(III) actinides and lanthanides from a dilute mineral acid by representatives of these two classes of extractants in toluene diluent may be represented, respectively, as:where the subscripts A and O refer to mutually equilibrated aqueous and organic phases. However, in an alcohol diluent the H2Y extractants appear to be monomeric, and they extract M+3cations with a third-power extractant dependency.In toluene diluent, the HY extractants function as dimers, the extractant dependencies for selected M+2and M+4cations being:(2-power); Ca+2, Sr+2, Ba+2(2.5-power, 3-power); Th+4(3-power). In certain systems, Th+4is extracted as a species containing one or two nitrate groups.Structures of the extracted species are postulated.