The melting behaviors of gel-spun, ultra-high molar mass polyethylene fibers have been investigated using standard differential scanning calorimetry and wide-angle X-ray diffraction. For laterally constrained fibers the standard DSC exhibits the usually reported two melting peaks (one at about 415 K and one at about 423 K), a glass transition (at about 275 K) and an unusually large and reversible heat capacity as one approaches the melting peaks (starting at about 350-370 K). Earlier observations that sometimes only the lower melting endotherm is retained, are proven to be of fibers free of longitudinal and lateral constraints. Up to the beginning of major melting, all fibers are mainly orthorhombic. The orthorhombic-hexagonal transition is proven to occur only for the constrained fibers, implying that the hexagonal phase is a non-equilibrium phase, usually not present in the initial fibers. An oriented, intermediate phase, discovered by solid state NMR and full-pattern X-ray analysis, gives rise to a substantially broadened glass transition that reaches to about 300 K, compared to the glass transition of the amorphous polyethylene with a midpoint at 237 K and end at 250 K. Published by Elsevier Science Ltd.