Excess molar enthalpies, measured at 298.15 K in a flow microcalorimeter, are reported for tetrahydrofuran + hexane + decane and for tetrahydrofuran + cyclohexane + decane. Smooth representations of the results are described and used to construct constant excess molar enthalpy contours on Roozeboom diagrams. The latter are compared with diagrams obtained when the model of Liebermann and Fried is used to estimate the excess molar enthalpies of the ternary mixtures from the physical properties of the components and the excess molar enthalpies of their binary mixtures.