On the basis of simulations of water radiolysis, it has been postulated that the yield of OH radicals, which become homogeneously distributed, is dependent upon energy when irradiated with low-energy photons. The aim of this study is to determine experimentally the dependence of the yield of OH radicals, which escape intratrack recombination, on photon energy of incident radiation using plasmid DNA as a probe. The yields of single strand breaks (ssb) induced in plasmid DNA (pUC18) when irradiated with photons varying in energy from 0.28 keV to 1.25 MeV was determined and, when normalized relative to the yield for Co-60 irradiation, may be used as a measure of the yield of OH radicals escaping radiation tracks. As the photon energy decreases from 1.25 MeV to 1.5 keV the OH radical yield decreases from 0.290 to 0.072 mu mol J(-1), respectively, in line with an increased ionization density of the radiation and hence an increasing probability of radical recombination. However, with a further decrease in photon energy from 1.5 to 0.28 keV there is an upturn in OH radical yields. Carbon-K X-rays are found to have a significantly higher yield of 0.257 mu mol J(-1) than that associated with the higher energies. The experimental dependence is compared with a number of theoretical calculations, which predict an upturn in OH radical yields between 0.1 and 1 keV. Such a dependence of OH radical yields on energy provides experimental data suitable for direct comparison with simulations, aiding their refinement and development.