Using a combination of differential scanning calorimetry and quasi-isothermal temperature-modulated calorimetry, we investigated the temporal evolutions of the melting temperature, degree of crystallinity, and excess heat capacity during crystallization of linear polyethylene and low styrene content ethylene-styrene copolymers. Describing isothermal crystallization as the succession of three stages (primary, mixed and secondary crystallization stages), we established the following correlations: (1) the evolution of the melting temperature with time parallels that of the degree of crystallinity, (2) the excess heat capacity increases linearly with degree of crystallinity during the primary stage, reaches a maximum during the mixed stage, and decays during the secondary stage, (3) the rate of decay of the excess heat capacity parallels the rate of secondary crystallization, and (4) the rates of shift of the melting temperature and decay of the excess heat capacity lead to apparent activation energies that are very similar to these reported for the crystal a, relaxation by solid-state NMR, dynamic mechanical, and dielectric spectroscopies. Strong correlations in the time domain for secondary crystallization by lamellar thickening and evolution of the excess heat capacity suggest that the reversible crystallization/melting phenomenon is associated with molecular events in the melt-crystal fold interfacial region. Specifically, we conclude that the excess heat capacity observed during the high-temperature crystallization of linear polyethylene and low styrene content copolymers is most likely to originate from the segmental processes in the crystal/melt fold region that have been discussed by Fischer, Mansfield, and Strobl. These studies also provide preliminary indications that the excess heat capacity observed during crystallization at lower temperatures in the case of ethylene copolymers of high comonomer content is consistent with the lateral surface model proposed by Wunderlich.