Density functional theory (BPW91/TZ2P) is used to explore the nature of cation-cation interactions (CCls) that exist between two actinyl cations in solution. Solvation, which is modeled using COSMO, favors the complexes (ONpO-ONpO)(2+) and (ONpO-OUO)(3+) over separated NpO2+(aq) and UO22+(aq) cations because of the quadratic dependence of solvation on charge. For (OUO-OUO)(4+), solvation effects, even though very large, are unable to overcome intrinsic electrostatic repulsion between the units. The actinyl-actinyl complexes are T-shaped, with the oxygen of one unit coordinated to the actinide metal of the other unit. The association free energies of (ONpO-ONpO)(2+) and (ONpO-OUO)(1+) are calculated as -42.1 and -29.2 kcal/mol. Explicit consideration of the first solvation shell at the B3LYP/LANL2DZ level suggests that the free energies of binding may be overestimated. The Hg-2(2+) dication, though not considered a "traditional" CCl, is very similar to the actinyl-actinyl interaction. The binding free energy of Hg-2(2+) in solution is calculated as -16.0 kcal/mol.