Vapor-liquid equilibrium at 94 kPa has been determined for the ternary system methyl 1,1-dimethylethyl ether (MTBE) + hexane + octane. The system deviates positively from ideality, and no azeotrope is present. The ternary activity coefficients and the boiling points of the system have been correlated with the composition using the Redlich-Kister, Wilson, NRTL, UNIQUAC, UNIFAC, and Wisniak-Tamir relations. Most of the models allow a very good prediction of the activity coefficients of the ternary system from those of the pertinent binary systems.