The change in the solid-state proton NMR Free Induction Decay signal of a polyethylene sample while heating from 360 to approximately 400 K was investigated. The crystallinity, as determined by model fit, decreases approximately linearly with increasing temperature up to 393 K. At higher temperature, the crystallinity decreases faster with increasing temperature. Within the temperature region investigated, three different "phases" are identified. The change in the relative distribution of these phases versus temperature is explained quantitatively by a phase-equilibrium model. Spin-spin relaxation time measurements and second moment calculations are presented, which gives information about molecular motion within these phases.