The concepts of local compositions around a solute and preferential solvation of a solute are defined in terms of the Kirkwood-Buff integrals. The difference between the local and the bulk composition is a measure of the preferential solvation of a solute with respect to the various components of the solvent. A statistical mechanical theory is developed that leads to simple relationships between local compositions and experimentally measurable quantities. Some preliminary results on preferential solvation of methane in mixtures of water-ethanol and water-p-dioxane are presented.