Polyethylene single-crystal mats 20 μm thick were heated to the annealing temperatures (120–132 °C) in less than 1 s. The Cornell High Energy Synchrotron Source (CHESS) and a TV detector system were used to obtain wide- and small-angle x-ray scattering patterns from these mats during annealing with a time resolution of 0.3 s. Both whole polymer and a molecular weight fraction showed rapid melting and recrystallization, with a minimum crystallinity at 2–4 s after reaching the annealing temperature. Recrystallization was essentially complete in 60–90 s and most curves could be fitted to the Johnson–Mehl–Avrami equation with exponent n=1. Two-thirds of the material could be molten before there was any loss of the original crystal orientation. The increase in long period was discontinuous at high annealing temperature. The original small-angle long-spacing peak quickly lost intensity as crystallinity fell and a new peak appeared at larger long spacing. This new long spacing increased with time for the whole polymer and was constant for the fractionated material. At lower annealing temperatures the amount melting was small and the small-angle intensity and long period increased continuously.