To make clear the action of organic solvent in solvent-assisted dyeing process, the surface tension of an aqueous solution of benzyl alcohol and acid dye (C. I. Acid Orange 7 or C. I. Acid Yellow 36) mixture was measured as a function of the total molarity (m) and the composition (X₂) of the mixture at 298.15 K under atmospheric pressure. By applying Motomura's thermodynamic equations to the experimental results, the total surface densities (_??_^H) and the composition (X^H₂) of the mixed adsorbed-film were evaluated numerically. The results indicated that dye in the mixed adsorbed-film is richer than in the mixed bulk solution, when the mixed dye is more surface active than benzyl alcohol. Accordingly, these results showed that, contrary to Giles's description (the presence of the organic solvent depresses the concentration of dye at the interface with air.), the presence of benzyl alcohol increases the concentration of dye at the interface with air, and most probably with wool or nylon fiber. It is thought that this effect occurred by benzyl alcohol is one of the prime factors which accelerate dye-diffusion into fiber.