Vapor-liquid equilibrium at 94 kPa has been determined for the systems hexane + heptane and methyl 1,1-dimethylethyl ether (MTBE)+hexane+heptane. The binary system exhibits regular, almost ideal behavior, and no azeotrope is present in the ternary system. The data were correlated by the Redlich-Kister, NRTL, Wilson, UNIQUAC, and Wisniak-Tamir equations and the appropriate parameters are reported. The models allow prediction of the activity coefficients of the ternary system from those of the pertinent binary systems.