Microcalorimetric measurements of excess molar enthalpies, at 298.15 K, are reported for x(1)(CH3)(3)CCH2CH(CH3)(2) + x(2)c-(CH2)(6) + x(3)CH(3)(CH2)(6)CH3. A smooth representation of the results is presented and used to construct constant excess molar enthalpy contours on a Roozeboom diagram. It is shown that good estimates of the ternary enthalpies can be obtained from the Flory theory, using only the physical properties of the components and their binary mixtures.