We report molecular dynamics (MD) simulations of aqueous solutions of Keggin heteropolyacids XWn– nH3O+ (XWn– = [PW12O40]3–, [SiW12O40]4–, and [AlW12O40]5–), the aim of which was to investigate whether the anions increasingly repulse each other in water when their charge increases from –3 to –5. The MD and PMF (potential of mean force) results indicate that this is not the case. All three anions are found to display short XWn–···XWn– contacts (“dimers”), the amount of which increases at increasing concentrations. However, the proportion of dimers follows the sequence SiW4– < PW3– < AlW5–, which is unexpected from interanionic Coulomb forces. The least-charged anions are mainly attracted by hydrophobic forces, whereas the most charged AlW5– anions afford the highest proportion of dimers through specific interactions with H3O+ counterions. Similar features are observed in two series of fictitious analogues of XWn– (n = 3 to 5): the Sn– spherical anions, and the XWn+ cations (with all atomic charges of XWn– inverted and “H3O–” as counterions). The key role of counterion condensation on the stabilization of the most negatively charged dimers is further demonstrated by comparing Li+ salts to Na+ and Cs+ salts of the different XWn– anions: anion–anion dimerization is found to be highest between AlW5– anions and in the presence of the “hardest” counterions (Li+). Likewise, with Eu3+ multivalent AlW5– counterions condense into a concentrated ionic domain in water. Finally, all three types of XWn– anions are found to adsorb at the aqueous interface with graphite. These findings are important for understanding processes like dimerization, condensation, third phase formation, and the assembly of polyoxometalates and macroions in water or at aqueous interfaces.