Microcalorimetric measurements of excess enthalpies at the temperature 298.15 K are reported for the two ternary mixtures (methyl 1,1-dimethylpropyl ether + 2,3-dimethylbutane + decane) and (methyl 1,1-dimethylpropyl ether + cyclohexane + decane). Smooth representations of the results are presented and used to construct constant-enthalpy contours on Roozeboom diagrams. Comparisons of the experimental results with estimates based on the Flory theory of mixtures are also described.