Calorimetric measurements of excess enthalpies are reported for the ternary system pentyl ethanoate + 1,2-dimethylbenzene + n-nonane and for binaries pentyl ethanoate + 1,2-dimethylbenzene at 298.15 and 318.15 K and pentyl ethanoate + n-nonane at 318.15 K. The results, together with those previously published for constituent binaries, were well correlated with a modified Redlich-Kister equation. The ternary excess enthalpies at 318.15 K were considered together with the excess Gibbs energies calculated by the Wilson equation with temperature dependence parameters.