A simple two-state structural model Of Solute hydration has been developed. In this model, both water in the bulk, state and water of solute hydration are assumed to consist of two structural specie,s: a high density/high enthalpy species, structurally similar to ice III, and a low density/low enthalpy species, structurally similar to ice I. It is assumed that structural and thermodynamic distinctions between bulk and hydration water originate solely from the differential fractional composition, whereas the two structural species and thermodynamic parameters associated with each species are identical for bulk and hydration water, This model has been used in conjunction with volumetric data reported in the literature to analyze the hydration properties of charged, polar, and nonpolar groups at 25 degreesC. The equilibrium between the two structural species of water of hydration of charged and polar groups is shifted toward the high density/high enthalpy species. In contrast, the equilibrium between the two species of water solvating nonpolar groups is shifted toward the low density/low, enthalpy species. Solvent reorganization was found to be thermodynamically unfavorable for any atomic group independent of its chemical nature. However, the enthalpy, entropy, and heat capacity of solvent reorganization are strongly dependent on the chemical nature of solvent exposed group. In the aggregate, Our results provide foundation for more reliable interpretations of thermodynamic data in terms of hydration. In addition, these results underscore the importance and potential usefulness OF combining volumetric and calorimetric data for a more complete thermodynamic description of microscopic events, in particular, solute hydration.