Ultra high molecular weight polyethylene has been irradiated in air and aged for 168 months. On melting and recrystallisation in the differential scanning calorimeter a secondary crystallisation event is observed around 80 degrees C. This event has been studied using isothermal crystallisation and the results analysed using the Avrami equation and the Lauritzen-Hoffman approach. This suggests that during this event growth occurs through regime II kinetics whereby large numbers of surface nuclei form on the substrate, with multiple nucleations acts commencing before the previous ones have finished. It is postulated that this secondary crystallisation event involves the development of a diffuse semi-ordered interface between the well-developed lamellae and the amorphous phase. (c) 2005 Elsevier Ltd. All rights reserved.