To examine the effect of solvent environment on protein salt-bridges, we performed high-level ab Initio molecular orbital calculations in the gas phase and in three different solvents on a salt-bridge as modeled by formate and guanidinium ions. The energy difference between the neutral hydrogen-bonded complex and the zwitterionic form and the interconversion barrier between them are investigated in detail at RHF/6-31G*,RHF/6-311+G** MP2/6-31G*, and MP2/6-311+G** levels. In the gas phase, the neutral conventional hydrogen-bonded complex is predicted to be favored at all four levels of theory and there is a small barrier for the interconversion. In a nonpolar, hydrophobic solvent like CCl4, the energy difference between these two forms is small and the barrier that separates them is also low, but the neutral hydrogen-bonded complex still seems to be slightly favored. However, in polar solvents like DMSO and water, the zwitterionic form becomes much more favored. In polar solvents, the barrier for conversion of the neutral hydrogen-bonded form to the zwitterionic form is small at the Hartree-Fock level, but it disappears at the correlated level (MP2). The implication of these findings toward stabilizing an enzyme in nonaqueous solvents is briefly discussed.