The melting and crystallization of gel-spun, ultrahigh molar mass polyethylene (UHMM-PE) fiber is analyzed with both, standard differential scanning calorimetry (DSC) and temperature-modulated DSC (TMDSC) to identify the differences to extended-chain crystals of polyethylene. The analyzed samples were studied before with respect to their molecular mobility, morphology, and crystal and phase structure. The UHMM-PE contains a mobile, oriented mesophase and only few folded chains. The truly amorphous fraction is about 3%. Reversing melting and crystallization is observed for these fibers. It increases in amount on recrystallization after melting. Difficulties in measuring the original fibers arose from the shrinkage of the fibers and a resulting deformation of the sample pan. The new data are compared to literature information on other polyethylenes and linked to reversible melting of decoupled segments of the polymer chains contained in small, nanometer-size phases within the crystalline phase and the oriented-intermediate fractions. (C) 2004 Elsevier B.V. All rights reserved.