Estimates of physical or thermodynamic properties of organic compounds are often based on a group contribution approach or other quantitative structure-property relationship. Polar functional groups introduce a perturbation to intramolecular distribution of electrons, resulting in violation of a group additivity assumption known as structural or proximity effects. We propose a method for quantitative evaluation of these effects from experimental data and present the results for polar functional group contributions and their structural/proximity effects calculated for several standard thermodynamic properties of hydration at ambient conditions and at high temperatures and pressures. Recommendations are provided for further developments of group contribution methods in aqueous solutions of organics.