Equilibrium properties of dilute binary fluid mixtures are studied in two-phase states on the basis of a Helmholtz free energy including the gradient free energy. The solute partitioning between gas and liquid (Henry's law) and the surface tension change Δγ are discussed. A derivation of the Gibbs law Δγ=-TΓ is given with Γ being the surface adsorption. Calculated quantities include the derivatives d T_c/dX and d p_c/dX of the critical temperature and pressure with respect to the solute molar

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... action X and the temperature-derivative (γ/ T)_ cx,p of the surface tension at fixed pressure p on the coexistence surface. Here (γ/ T)_ cx,p can be both positive and negative, depending on the solute molecular size and the solute-solvent interaction,and diverges on the azeptropic line. Explicit expressions are given for all these quantities in the van der Waals model.