The Kirkwood-Buff formalism was used to derive an expression for the composition dependence of the Henry's constant in a binary solvent. A binary mixed solvent can be considered as composed of two solvents, or one solvent and a solute, such as a salt, polymer, or protein. The following simple expression for the Henry's constant in a binary solvent (H-2t) was obtained when the binary solvent was assumed ideal: In H-2t = [In H-2,H-1(ln V - In V-3(0)) + In H-2,H-3(ln V-1(0) - ln V)]/ (ln V-1(0) - ln V-3(0)). In this expression, H-2,H-1 and H-2,H-3 are the Henry's constants for the pure single solvents 1 and 3, respectively; V is the molar volume of the ideal binary solvent 1-3; and V-1(0) and V-3(0) are the molar volumes of the pure individual solvents 1 and 3. The comparison with experimental data for aqueous binary solvents demonstrated that the derived expression provides the best predictions among the known equations. Even though the aqueous solvents are nonideal, their degree of nonideality is much smaller than those of the solute gas in each of the constituents. For this reason, the ideality assumption for the binary solvent constitutes a most reasonable approximation even for nonideal mixtures.