Proton magnetic resonance studies have been made to compare the molecular motions ascribable to the disordered regions of solution‐grown and melt‐grown specimens of solid polyethylene. Studies have been made of both the spin‐lattice and spin‐spin relaxations. Restricted motion attributable to defect regions develops in solution‐grown crystals as the temperature is raised, but the number of protons in motion changes reversibly with heating and cooling, provided the maximum temperature is below ∼100°C. With higher temperatures the motion is greater in abundance and broader in frequency distribution. The increase is permanent despite annealing. Segmental motion attributed to molecules within the crystal is seen in solution‐grown crystals in the interval 50–100°C, but occurs to a much smaller degree in melt‐grown and heat‐treated samples. A model for the observed behavior is proposed.