In the measurement of infinite-dilution activity coefficients for a multicomponent system by the gas stripping method, this paper shows a way to obtain more limiting data to improve the study of multicomponent phase equilibria. As an example, the infinite-dilution ternary activity coefficients gamma(A(B,C)(infinity)) for the acetonitrile-benzene-n-heptane system were determined at 318.15 K. Inasmuch as the solvents involved are often composed of two components, the double-cell technique (DCT) was employed to obtain steady solvent surroundings for the dilute solutes, which are apparently an essential condition for reasonable and meaningful measurements of infinite-dilution activity coefficients. The experimental gamma(A(B,C)(infinity)) values were correlated by use of the NRTL and UNIQUAC equations, and the vapor-liquid and liquid-liquid equilibria for the relevant system could be predicted.