(Vapour + liquid) equilibrium of {methyl 1,1-dimethylethyl ether (MTBE) + n-heptane + methanol}, (methyl 1,1-dimethylethyl ether (MTBE)+ methanol}, and (methanol + n-heptane) have been measured at T = 313.15 K. The data reduction by Barker's method provides correlations for the excess molar Gibbs energy G(m)(E) using the Margules equation for the binary systems, and the Wohl expansion for the ternary. The Wilson, NRTL, and UNIQUAC models have been applied successfully to both the binary and ternary systems reported here. Using literature data of the excess molar enthalpy H-m(E) for the same ternary mixture measured at T = 313.15 K, we have calculated the excess molar entropy S-m(E) for the system at the same temperature.