Grafting rates of gaseous butadiene to a range of morphological forms of gamma-irradiated polyethylene, including ultra-high-modulus fibres (UHMPE), have been measured in order to determine the availability of active free radicals over time at various temperatures. Blank experiments on unirradiated samples showed that monomer diffusion is not rate-controlling with film and natural draw ratio tapes, but is likely to be a major factor in the control of grafting rates in UHMPE fibres. Grafting rates from monomer loss/time experiments with irradiated samples indicate that grafting is always in competition with free radical self-annihilation, the extent being influenced by temperature, dose and morphology, including prior sample annealing. At lower temperatures, graft-active radicals are produced over long periods of time, eg close to linear grafting rates were monitored over 20 hours for PE tape at 0 à(50 kGy) and for gel-spun UHMPE at 40 à(100 kGy). At higher temperatures, grafting rates steadily decrease with time. Grafting rates are almost independent of irradiation dose in the early stages, however, the dose has an increasing positive influence as the reaction proceeds. At any given temperature and irradiation dose, the rates decrease in the series undrawn film; natural draw ratio tape; high draw ratio gel-spun fibre; high draw ratio melt-spun fibre. An analogy is drawn between these results and the optimum conditions required for improving the creep properties of PE tape and UHMPE fibres by acetylene-sensitized irradiation crosslinking.