Microcalorimetric measurements of excess molar enthalpies at 298.15 K are reported for the two ternary mixtures, methyl tert-butyl ether (MTBE) or di-isopropyl ether (DIPE) + n-octane + 2-methylpentane (2-MP). The excess enthalpies for DIPE + 2-MP are also reported. Smooth representations of the ternary enthalpies results are presented and used to construct constant excess molar enthalpy contours on Roozeboom diagrams. It is shown that good estimates of the ternary enthalpies can be obtained from the Liebermann and Fried model, using only the physical properties of the components and their binary mixtures.