Recent experimental and theoretical studies showed the preference of the hydronium ion for the vapor/water interface. To investigate the mechanism responsible for the surface propensity, of this ion, we performed a series of novel quantum chemical simulations combined with the theory of solutions. The solvation free energy of the H3O+ solute placed at the interface was. obtained as -97.9 kcal/mol being more Stable by 3.6 kcal/mol than that of the solute embedded in the bulk Further, we decomposed the solvation;free:.; energies into contributions from the water molecules residing in the oxygen and the hydrogen sides of the solute to clarify the origin of the surface preference. When the solute was displaced from the bulk to the interface, it was shown that the free energy contribution from the oxygen side is destabilized by similar to 10 kcal/mol because of a reduction of the number of surrounding solvent Water molecules. It was observed, however, that the free energy contribution due to the hydrogen side of the solute is unexpectedly stabilizing and surpasses the destabilization in the opposite side. We found that the stabilization in the hydrogen side originates from the solute solvent interaction in the medium range beyond the nearest neighbor. It was also revealed that the free energy contribution due to the solute's electronic polarization amounts to about the half of the total free energy change associated with the solute displacement from the bulk to the interface.