Purely organic radical ions dimerize in solution at low temperature, forming long, multicenter bonds, despite the metastability of the isolated dimers. Here, we present the first computational study of these π-dimers in solution, with explicit consideration of solvent molecules and finite temperature effects. By means of force-field and ab initio molecular dynamics and free energy simulations, the structure and stability of π-[TCNE]2 2− (TCNE=tetracyanoethylene) dimers in dichloromethane have been evaluated. Although the dimers dissociate at room temperature, they are stable at 175 K and their structure is similar to the one in the solid state, with a cofacial arrangement of the radicals at an interplanar separation of approximately 3.0 Å. The π-[TCNE]2 2− dimers form dissociated ion pairs with the NBu4 + counterions, and their first solvation shell comprises approximately 20 CH2Cl2 molecules. Among them, the eight molecules distributed along the equatorial plane of the dimer play a key role in stabilizing the dimer through bridging C−H⋅⋅⋅N contacts. The calculated free energy of dimerization of TCNE. − in solution at 175 K is −5.5 kcal mol−1. These results provide the first quantitative model describing the pairing of radical ions in solution, and demonstrate the key role of solvation forces on the dimerization process.