Isothermal crystallization studies of low-density polyethylene (LDPE) and high-density polyethylene (HDPE) using differential scanning calorimetry (DSC) were performed using different sample thicknesses to determine the effect of non-ideal heat-transfer. Polyethylene was chosen because of its importance, its extensive coverage in the literature, and its fast crystallization kinetics. Thermal gradients were found to significantly affect the measured crystallization exotherm; slower crystallization rates were observed for thicker samples measured at lower temperatures (greater supercoolings). Differences between different sample thicknesses disappeared at higher temperatures, consistent with finite heat-transfer rates being responsible for the effect. A power-compensation and a heat-flux DSC were used; these experiments also enabled the determination that the performance of the latter was acceptable for this study. Finally, thickness-independent Avrami parameters have been calculated. (c) 2006 Elsevier B.V. All rights reserved.