Results are presented of parallel calorimetric and dilatometric experiments on several meltcrystallized polyethylenes, with various thermal histories, over the range –35°→ liquidus. Enthalpic and weight fraction degrees of crystallinity are in good agreement up to 60°C, where direct comparison with n-alkane data is possible. Hence the interfacial surface enthalpy of the crystallites is small and there is evidence to show such behaviour continues to the melting point. The derived heat of fusion of perfectly crystalline polyethylene exhibits considerable curvature at low temperatures; at the melting point it is 307 J g–1, some 5 % above the highest previous estimate. The temperature dependence of the free energy of fusion indicates an extended chain melting point of 141 ± 1°C.