Hydroxylation reactions of aromatic compounds have been used to detect hydroxyl radicals produced by gamma irradiation and ultrasound. The present study investigated the suitability of terephthalic acid (THA) as a hydroxyl radical dosimeter for general use in biologically relevant reactions. Hydroxyl radicals were generated by: (1) irradiating THA with a 254 nm ultraviolet light; (2) irradiating with gamma rays from a cesium source; and (3) generating hydroxyl radicals with 1 mM H2O2 and 10 microM Cu+2. In each of the three experiments, a fluorescent product was generated which exhibited identical fluorescent excitation and emission spectra. THA is non-fluorescent, eliminating the problem of a high initial background. Because THA has four identical ring hydrogens, only one mono-hydroxylated isomer was formed. The hydrogen peroxide reaction was dependent on the presence of a metal and cupric ions were effective in enhancing the reaction. With a Cu+2 concentration of 10 microM, the reaction was linear between 0-30 mM H2O2. Catalase abolished the reaction at a concentration of 100 micrograms/ml and the effects could still be observed at 10 ng/ml, consistent with the very high rate at which catalase destroys hydrogen peroxide. Tertbutyl- hydroperoxide did not generate any fluorescence in this system which makes THA a very specific detector of hydroxyl radicals.