Isobaric vapor-liquid equilibria for 2-methoxy-2-methyl propane (MTBE) Jr 2-methyl-2-propanol + octane and the constituent binary system MTBE + 2-methyl-2-propanol have been measured at 101.3 kPa. The measurements were made in an equilibrium still with circulation of both the vapor and liquid phases. The ternary system of MTBE + 2-methyl-2-propanol + octane and the binary system of MTBE + 2-methyl-2-propanol are both non-azeotropic. The constituent binary systems of 2-methyl-2-propanol + octane and MTBE + octane, which were measured in our previous work [T. Hiaki, A. Taniguchi, T. Tsuji, M. Hongo, K. Kojima, J. Chem. Eng. Data, 41 (1996) 1087-1090; T, Hiaki, K. Tatsuhana, T, Tsuji, M. Hongo, J. Chem. Eng. Data (1998), in press] are also non-azeotropes. The experimental data of the binary systems were correlated with the non-random two-liquid (NRTL) equation. The NRTL equation yielded a good prediction of activity coefficients for the ternary system from the parameters of the correlated binary systems.