Temperature-modulated differential scanning calorimetric (TMDSC) measurements on nascent ultra-high molecular mass polyethylene (UHMMPE) in scanning as well as quasi-isothermal mode are presented. From these measurements, a special irreversible relaxation process in the pre-melting region can be separated, which only occur on the first heating of the nascent material. The frequency dependence of this process yield a characteristic time constant of 6.5 s (or 24 mHz) which fits with the alpha (2)-process known from mechanical measurements. Quasi-isothermal measurements show that there are two slow (exponential) relaxation processes with time constants of 2-5 and several hundred minutes, respectively. The latter is only found in the first run of nascent UHMMPE and seems to be connected with irreversible structural changes (crystal thickening and ordering). The low activation energy (ca. 50 kJ mol(-1)) points to a chain diffusion process rather than melting and crystallization.