The production of molecular hydrogen in the radiolysis of high-density polyethylene by gamma rays and heavy ion beams has been investigated. Only the slightest increase in the radiation chemical yield of 3.1 molecules/100 eV was found from gamma rays to protons of 5-15 MeV. A gradual increase in yield was observed on further increasing the linear energy transfer of the incident particles. This increase amounted to almost a doubling in the hydrogen yield from 10 eV/nm protons to about 800 eV/nm carbon ions. The exact reason for the increase in molecular hydrogen yield is uncertain, but it may involve reactions of excited states or enhanced combination reactions of carbon-centered radicals, thereby allowing more hydrogen to escape the particle tracks. Diffusion from the bulk material was found to have a dominant role on the observed dynamic profiles of hydrogen gas. A simple one-dimensional diffusion model was used to estimate the diffusion constant of hydrogen in polyethylene to be 2.2 x 10(-6) cm(2)/s.