Vapor-liquid equilibrium (VLE) and excess enthalpy (HE) data of binary and ternary mixtures involving 1,1-dimethylethyl methyl ether (tert-butyl methyl ether or MTBE) or 1,1-dimethylpropyl methyl ether (tert-amyl methyl ether or TAME) and alkanes, olefins, benzene, and alcohols have been studied using the UNIQUAC model, the original UNIFAC group contribution model, and the modified UNIFAC models of Lyngby and Dortmund. Values for the binary parameters required in the UNIQUAC model are obtained from binary VLE or H-E data. Binary VLE data are predicted using the UNIQUAC equation with parameters obtained from H-E data and the UNIFAC and modified UNIFAC models. Ternary VLE data are predicted using the UNIQUAC equation with parameters obtained from binary H-E or VLE data and the UNIFAC and modified UNIFAC models. Accurate ternary data are obtained using the UNIQUAC equation with parameters determined from binary VLE data. The modified UNIFAC (Dortmund) model is shown to provide good predictions of binary and ternary VLE data and can be recommended for those cases in which the related binary VLE data are not available.