Statistical mechanics provides the basis for a rigorous theory of molecules and molecular complexes in chemically inert media. It is found that the free energy of solvation may be calculated, provided the distribution function for the solvent molecules around the solute molecules is known. A relation is deduced between the volume change on dissolving, the free energy of solvation, and the isothermal compressibility of the pure solvent. The general equations are applied to a solution of solid spheres in a structureless medium. The theoretical deductions are found to be in qualitative agreement with the thermodynamic solvation parameters for oxygen and argon in inert solvents.