Measurements of excess molar enthalpies at the temperature 298.15 K, in a flow microcalorimeter are reported for the ternary mixture {x(1)(CH3)(3)CCH2CH(CH3)(2)(+) x(2)c-(CH2)(6) + (1 - x(1) - x(2))CH3(CH2)(10)CH3} A Smooth representation of the results is described and used to construct constant-enthalpy contours on a Roozeboom diagram. It is shown that useful estimates of the ternary enthalpies can be obtained from the Flory theory using only the physical properties of the components and their binary mixtures.