A new theory of electrolyte and nonelectrolyte solutions has been developed which, unlike the Debye-Huckel method applicable for small concentrations only, makes it possible to estimate thermodynamic properties of a solution in a wide range of state parameters. One of the main novelties of the proposed theory is that it takes into account the dependence of solvation numbers upon the concentration of solution, and all changes occurring in the solution are connected with solvation of the stoichiometric mixture of electrolyte ions or molecules. The present paper proposes a rigorous thermodynamic analysis of hydration parameters of solutions. Ultrasound and densimetric measurements in combination with data on isobaric heat capacity have been used to study aqueous solutions of electrolytes NaNO(3), KI, NaCl, KCl, MgCl(2), and MgSO(4) and of nonelectrolytes urea, urotropine, and acetonitrile. Structural characteristics of hydration complexes have been analyzed: hydration numbers h, the proper volume of the stoichiometric mixture of ions without hydration shells V(2h), compressibility beta(1h), and the molar volume of water in hydration shells V(1h), their dependencies on concentration and temperature. It has been shown that for aqueous solutions the electric field of ions and molecules of nonelectrolytes has a greater influence on the temperature dependence of the molar volume of solution in hydration shells than a simple change of pressure. The cause of this effect may be due to the change in he dielectric permeability of water in the immediate vicinity of hydrated ions or molecules, The most studied compounds (NaCl, KCl, KI, MgCl(2)) have been studied in a wider range of solute concentrations of up to 4-5 mol/kg. Up to the complete solvation limit (CSL), the functions V(1h) = f(T) and beta(1h) = f(T) are linear with a high correlation factor, and the dependence Y(K,S) = f(beta(1)V(1)*) at all investigated concentrations of electrolytes and nonelectrolytes up to the CSL enables h and beta(h)V(h) to be determined on the basis of relationships obtained in the study. The behavior of nonelectrolyte solutions is no different from that of electrolyte solutions, although it is possible to trace the difference between hydrophobic and hydrophilic interactions.