Insulin dimerization and aggregation play important roles in the endogenous delivery of the hormone. One of the important residues at the insulin dimer interface is Phe(B24), which is an invariant aromatic anchor that packs toward its own monomer inside a hydrophobic cavity formed by Val(B12), Leu(B15), Tyr(B16), cys(B19), and Tyr(B26). Using molecular dynamics and free-energy simulations within explicit solvent, the structural and dynamical consequences of mutations of Phe at position B24 to glycine (Gly), alanine (Ala), and D-Ala and the des-PheB25 variant are quantified. Consistent with experiments, it is found that the Gly and Ala modifications lead to insulin dimers with reduced stability by 4 and 5 kcal/mol from thermodynamic integration and 4 and 8 kcal/mol from results using molecular mechanics-generalized Born surface area, respectively. Given the experimental difficulties to quantify the thermodynamic stability of modified insulin dimers, such computations provide a valuable complement. Interestingly, the Gly mutant exists as a strongly and a weakly interacting dimer. Analysis of the molecular dynamics simulations shows that this can be explained by water molecules that replace direct monomer-monomer H-bonding contacts at the dimerization interface involving residues B24 to B26. It is concluded that such solvent molecules play an essential role and must be included in future insulin dimerization studies.