The melting and crystallization of extended-chain crystals of polyethylene are analyzed with standard differential scanning calorimetry and temperature-modulated differential scanning calorimetry. For short-chain, flexible paraffins and polyethylene fractions up to 10 nm length, fully reversible melting was possible for extended-chain crystals, as is expected for small molecules in the presence of crystal nuclei. Up to 100 nm length, full eutectic separation occurs with decreasingly reversible melting. The higher-molar-mass polymers form solid solution crystals and retain a rapidly decreasing reversible component during their melting that decreases to zero about 1.5 K before the end of melting. An attempt is made to link this reversible melting to the known, detailed morphology and phase diagram of the analyzed sample that was pressure-crystallized to reach chain extension and practically complete crystallization.