In an ongoing effort to determine the relationship between structure and function in diphosphonic acids, the aggregation and solvent extraction chemistry of four novel silicon-substituted diphosphonic acids, P,P'-di[3-(trimethylsilyl)-1-propyl] propylene- (H2DTMSP[PrDP]), butylene-(H2DTMSP[BuDP]), pentylene- (H2DTMSP[PDP]), and hexylene diphosphonic acid (H2DTMSP[HDP]), were investigated. Vapor pressure osmometry indicates that H2DTMSP[PrDP] and H2DTMSP[PDP] are dimeric in toluene at 25degreesC, while H2DTMSP[BuDP] and H2DTMSP[HDP] are primarily trimeric and hexameric, respectively. The solvent extraction of selected alkaline earth cations (Ca, Sr, and Ba) and representative tri-, tetra-, and hexavalent actinides [Am(III), Th(IV), and U(VI)], from nitric acid solutions into o-xylene solutions of the ligands has been studied and compared to results reported previously for some analogous di(2-ethylhexyl)-substituted. diphosphonic acids. Differences observed in the magnitude of distribution ratios, based on the number of bridging methylene groups, have been explained considering factors such as ligand aggregation, chelate ring size, P=O basicity (qualitatively determined by P-31 nuclear magnetic resonance and infrared spectroscopy), and the relative acidities of the ligands.