As a model for the direct effect of ionizing radiation, time-resolved changes of light-scattering intensity (LSI) of aqueous, oxic solutions of single-stranded DNA samples (calf thymus, Micrococcus lysodeikticus, Clostridium perfingens), polyC, polyU, and polyA at pH 7.5-8 have been studied following pulse irradiation with 193-nm laser light. A comparison of LSI changes (an index of single-strand break formation) with transient optical absorption changes (an index of nucleic acid base radical chemistry) following pulsed 193-nm irradiation shows that strand breakage occurs at rates similar to those for the decay of the nucleic acid base radical species for DNA, polyC, and polyU but not for polyA. With the exception of polyA, 193-nm light leads to a reduction in the LSI over similar to 0.2 s, indicating that strand breakage occurs by a nucleobase radical-mediated process, whereby the nucleobase radical site is transferred to the sugar moiety. With polyU and polyC, significant changes in LSI also occur on a much faster time scale, (within 3 ms). In DNA, the one electron oxidized sites become localized predominantly at guanine and the resulting one electron oxidised guanine radicals lead to strand breakage in competition with major, non-strand-breakage pathways.