An extension of the mixture-model approach to the theory of liquid water is developed to include aqueous solutions of nonelectrolytes. The Kirkwood-Buff theory of solution is employed to obtain a general and exact expression for the stabilization effect induced by the solute. This relation is applied, in the framework of a two-structure model, to obtain further insight into the molecular origin of some anomalous thermodynamic properties of aqueous solutions. The generalized continuous mixture-model formalism is also extended to solutions. It is demonstrated that current concepts such as structural changes in the solvent are strongly dependent on the particular classification procedure adopted to construct the mixture model.