The free energy differences between the E and Z conformers of formic acid and methyl formate, and the keto-enol tautomerization of 2,4-pentanedione in the gas phase and in organic solution, acetonitrile, are studied by ab initio molecular orbital calculations at the HF/6-31G(d), B3LYP/6-31G(d), and MP2/6-31+G(d) levels. The solvation free energies are calculated by using the continuum CPCM model. In addition, treating the solute with one and two explicit water molecules in the continuum model is pursued. Solvent effects on the conformational preference of the conformers will be discussed.