Solvent extraction reagents containing the diphosphonic acid group exhibit an extraordinary affinity for tri-, tetra-, and hexavalent actinides. Their use has been considered for actinide separation and preconcentration procedures. Solvent extraction data obtained with P,P'-di(2-ethylhexyl) methane-, ethane-, and butanediphosphonic acids exhibit features that are difficult to explain without knowledge of the aggregation state of the extractants. Information about the aggregation of the dialkyl-substituted diphosphonic acids in aromatic diluents has been obtained using the complementary techniques of vapor pressure osmometry, small-angle neutron scattering, infrared spectroscopy, and molecular mechanics. The results from these techniques provide an understanding of the aggregation behavior of these extractants that is fully compatible with the solvent extraction data. The most important results and their relevance to solvent extraction are reviewed in this paper.