Using a dynamic flow method, the supercritical carbon dioxide (SC-CO2) solubilities of two series of symmetrically substituted alkylenediphosphonic acids, bearing 2-ethylhexyl and 3-trimethylsilyl-1-propyl ester groups, respectively, were determined as a function of the number of methylene groups separating the two phosphorus atoms. An even-odd effect, similar to that observed previously for the aggregation of these compounds in nonpolar diluents, was observed, with compounds that form more highly aggregated species in nonpolar diluents exhibiting lower solubility in SC-CO2. Differences in the relative solubilities of analogous members of these series prompted the study of the SC-CO2 solubilities of symmetrically substituted methylenediphosphonic acids bearing seven- and eight-carbon ester groups of various degrees of branching to determine the relative importance of steric and electronic effects in determining SC-CO2 solubilities. When molecular connectivity indices were used to quantify the extent of branching in the ester groups, a remarkable correlation between these molecular descriptors and SC-CO2 solubility was observed.