Infinitely dilute hydrocarbon/water and alcohol/water systems were examined, with the objective to gather information about the size of the volume which is affected by the presence of a solute molecule and the interaction energy parameter between solute and solvent. First, an expression for the local composition at infinite dilution was obtained on the basis of the Kirkwood-Buff theory of solution. Second, equations for the activity coefficients at infinite dilution were derived using a modified Flory-Huggins equation for the excess free energy. In this modified expression, the molar fractions in the volume fractions were replaced by the local compositions provided by the first step. Finally, an additional expression for the local molar fraction was selected, which was coupled with that obtained on the basis of the Kirkwood-Buff approach. Experimental data regarding the activity coefficient at infinite dilution combined with the above equations allowed to obtain the values of the correlation volume (volume which is affected by a single solute molecule) and the interaction energy parameter between solute and solvent. The thickness of the layer of solvent influenced by the presence of a solute molecule was found to be equal to several molecular shells of water molecules (from 4 for propane to 7-8 for dodecane).