Superheating and double-melting endotherms are the characteristic melting behaviors of gel-spun ultra-high modulus and molecular weight polyethylene (UHMWPE) fibers in differential thermal analysis (DTA). A mostly orthorhombic structure with very little amorphous content is indicated by wide angle x-ray scattering (WAXS) data. The melting temperatures are elevated and believed to result from superheating and incomplete chain relaxation in the highly oriented and crystalline structure. The melting behavior of the fibers is shown to be strongly affected by changes in polymorphic transformations and in intercrystalline disordered domains. Compression in the direction perpendicular to fiber axis causes significant increases in the 110 and 200 dimensions of the orthorhombic structure. Such lateral compression increases monoclinic forms and perhaps amorphous content, and decreases the degree of transformation to hexagonal phase. Superheating, which is related to the intercrystalline stress, can be reduced by the presence of interacting solvents such as trichlorbenzene.