Isothermal vapour-liquid equilibria have been determined for the partially miscible ternary n-decane-2-methylnaphthalene-tetraethylene glycol in the miscible region and its two constituent miscible binaries at 160-degrees-C. The mutual solubilities of n-decane-tetraethylene glycol have been determined at 110-degrees-C for computing binary NRTL parameters. These binary parameters were used to predict and compare the ternary experimental data. The vapour-phase compositions for the ternary and the two binaries have been correlated by the NRTL, Wilson, UNIFAC, and three-constant Redlich-Kister models.