Excess molar enthalpies, measured at 298.15 K in a flow microcalorimeter, are reported for the ternary mixtures (ethanol + methyl tert-butyl ether + n-octane) and (1-propanol + methyl tert-butyl ether + n-octane). Smooth representations of the results are described and used to construct constant enthalpy contours on Roozeboom diagrams.