The observation of chain conformation and mobility in polyethylene by solid-state C-13 magic angle spinning (MAS) nuclear magnetic resonance (NMR) permits unambiguous identification and quantitative analysis of an intermediate phase. The carbon-carbon bonds in the intermediate phase adopt, on the average, an all-trans conformation and are more mobile than in the crystalline state (room temperature rate of reorientation approximate to 10(7) Hz). Comparisons of crystallinities by differential scanning calorimetry, wide-angle x-ray diffraction, and NMR support the high orientation of the intermediate phase and suggest a lower heat of fusion than for the crystals. Results from C-13 spin-lattice relaxation and H-1 spin diffusion show that the mass fractions are approximate to 20% and the domain sizes approximate to 36 Angstrom. Both change with crystallization and annealing conditions.