The inapplicability of the original statement of Raoult's law to binary solvent mixtures has been known for many years. An appropriate form for binary solvent mixtures is developed from the fundamental thermodynamic concept of fugacity, and is shown to be quantitatively applicable to the water+tetrahydrofuran solvent system and qualitatively applicable to the results which have been obtained for other aqueous organic solvent mixtures. © 1984 Plenum Publishing Corporation.