The thermodynamics of association of several binding systems, including protein—carbohydrate, small molecule—small molecule, protein—peptide, and protein—nucleic acid, were evaluated calorimetrically in light and heavy water. In every case, the enthalpy of binding in D2O was decreased relative to that in H2O: the differences range from 400 to 1800 cal mol-1. A compensating change in AS left the free energy of binding virtually unchanged in each case. A strong correlation between the differential enthalpy of binding and ACp for binding was observed, with a slope of 5 K. An analysis of the observed effect utilizing a Born-Haber thermodynamic cycle shows that the measured decrease in enthalpy represents approximately 10% of the binding enthalpy arising from solvent reorganization. For the range of systems investigated, solvent reorganization provides 25—100% of the observed enthalpy of binding. The thermodynamics of association of several binding systems, including protein—carbohydrate, small molecule—small molecule, protein—peptide, and protein—nucleic acid, were evaluated calorimetrically in light and heavy water. In every case, the enthalpy of binding in D2O was decreased relative to that in H2O: the differences range from 400 to 1800 cal mol-1. A compensating change in ΔS left the free energy of binding virtually unchanged in each case. A strong correlation between the differential enthalpy of binding and ACp for binding was observed, with a slope of 5 K. An analysis of the observed effect utilizing a Born-Haber thermodynamic cycle shows that the measured decrease in enthalpy represents approximately 10% of the binding enthalpy arising from solvent reorganization. For the range of systems investigated, solvent reorganization provides 25—100% of the observed enthalpy of binding. © 1994, American Chemical Society. All rights reserved.