In order to investigate the effect of the changing linear energy transfer along an alpha particle track in the radiolysis of water, aqueous solutions of 0.1N H/sub 2/SO/sub 4/ containing either ferrous, ceric, or ceric and thallous ions were irradiated by a flat external alpha source. G(Fe/sup +3/) and G(Ce/sup +3/) were plotted against the length of an alpha track in the solution. For comparison, the linear energy transfer was also plotted for various distances from the end of the track. Beta irradiation of the solutions was also studied. When D/sub 2/O was used an isotopic effect was observed which first increased with increasing mean linear energy transfer, then passed through a maximum, and decreased in the region of mean linear energy transfer corresponding to low energy alpha particles. (M.C.G.)