Complexes between Am{sup 3+} or Eu{sup 3+} and phosphonoacetic acid differ in relative stability in accord with the electrostatic model of cation binding. The smaller Eu{sup 3+} cation forms stronger complexes with PAA than the larger Am{sup 3+} cation. The observed metal complexes in the acid range from 0.005 M to 0.02 M (at I= 0.5 M) are Eu(HL){sup +}, Eu(H{sub 3}L){sub 2}{sup +}, Eu(HL){sub 2}{sup {minus}} and Am(H{sub 2}L){sup 2+}, Am(HL){sup +}, Am(H{sub 2}L){sub 2}{sup +}. When used as a holdback reagent, PAA slightly enhances the separation of Am/Eu when used with sulfonic acids or CMPO/nitrate, but reduces separation efficiency with HDEHP. In a CMPO/SCN{sup {minus}} extraction system which favors extraction of Am over Eu, addition of PAA increases the separation efficiency by a factor of 2-3 at 0.3 M PAA/0.5 M SCN{sup {minus}}. The calculated stability constants can be used to explain the separation factors, but do not always accurately predict metal distribution ratios in the CMPO systems, implying that there are details of this system which have not been fully elucidated
