Octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide (OφD(iB)CMPO) and extracted praseodymium form large elongated polymers whose sizes and shapes were measured by small-angle neutron scattering (SANS). The particles were 17-40 Ȧ in diameter and up to 500 Ȧ long. The polymers are produced at low 0φD(iB)CMPO/Pr ratios, and their sizes decrease as OφD(iB)CMPO/Pr ratio increases. A model for the polymers that invokes bridging of successive praseodymium atoms by the two functional groups of 0φD(iB)CMP0 is supported by: (a) the formation of polymers by two other Afunctional extractants, whereas a comparable monofunctional extractant fails to form polymers, and (b) the long and thin shape of most of the observed polymers, exhibiting a radial extension that is consistent with the model. Four comparable carbamoyl-alkyl-phosphoryl extractants have been examined by SANS for their capacity to form polymers with praseodymium in C6D6. Consistent with the model, the extractants, having the more basic amidic carbonyl moiety form polymers; one with less a basic carbonyl group does not form a polymer,