The study of the liquid-liquid equilibria in quaternary systems is not only of interest in order to provide necessary useful data in process design but also to check the thermodynamic models for the treatment of these data. In this work, experimental data of the quaternary system 1-octanol + 2-methoxy-2-methyl butane (tert-amyl methyl ether or TAME) + water + methanol at a temperature of 25 degrees C are presented, corresponding to a system with two partially miscible pairs (water + 1-octanol and TAME + water). The experimental data have been correlated using three equations, a modified Wilson, NRTL and UNIQUAC models to obtain the binary interaction parameters of these components. These data are also compared to those predicted using the UNIFAC method. The stability of the binary subsystems using a form of the Gibbs free energy in excess is also verified.