Intramolecular hydrogen bonding in monocations (called `internal solvation`) observed for bifimctional ligands with a rigid and flexible conformations cannot be neglected in the gas phase. It has strong influence on the thermodynamic parameters of simple proton-transfer reactions and tautomeric equilibria. As compared to monofunctional derivatives, the PA (or _PK_T) increase depends on the quality of functional group and geometry of the intramolecular H-bond. In bidentate ligands, the largest ΔPA values are observed for oxy functions separated by four carbons. An analysis of substituent effects in tautomerizing systems shows that the internal solvation in the gas phase has similar influence on the transmission of structural effects as the external solvation in solution.