Excess molar enthalpies, measured at 298.15 K in a flow microcalorimeter, are reported for the ternary system heptane (1) + ethanol (2) + 1,2-dimethoxyethane (3). A smooth representation of the results is described and used to construct constant-enthalpy contours on a Roozeboom diagram. A relative maximum of the excess molar enthalpy (1585 J.mol(-1)) is found where x(1) = 0.4047 and x(2) = 0.1862.