The temperature dependence of the structure of either cross-linked or non-cross-linked ultra-high-molecular-weight polyethylene (UHMWPE) fiber compacts was studied by synchrotron microbeam wide-angle X-ray scattering (WAXS), focusing on the fiber-fiber interface. The phase transition sequence is: melting of the monoclinic phase in the fiber skin, which was completed by 135 degreesC; melting of the unconstrained orthorhombic phase, by 152degreesC; melting of the constrained orthorhombic phase and a orthorhombic-hexagonal phase transition until 157degreesC; and gradual melting of the hexagonal phase, up to 165 degreesC. Cross-linking provides additional thermal stabilization.