The aggregation of several metal complexes formed during solvent extraction with P,P'-di(2-ethylhexyl) methanediphosphonic acid, H2DEH[MDP], in deuterated toluene, has been investigated by small angle neutron scattering (SANS). The SANS results show that the Ca(II)-, Al(ni)-, La(III)- and Nd(III)-H2DEH[MDP] compounds exhibit little or no tendency to aggregate. Uranyl complexes of H2DEH[MDP] at high metal loading form small aggregates with an aggregation number of approximately 14. This behavior is in sharp contrast with the Th(IV)-H2DEH[MDP] system which forms large aggregates in the organic phase under high metal loading conditions. The aggregation behavior of the various metal-H2DEH[MDP] compounds is discussed in light of the information obtained from earlier solvent extraction, vapor pressure osmometry, and infrared spectroscopy studies.