Objective To investigate whether ?OH radical species present in the acid oxidation-potential water (AOW). Methods Electron paramagnetic resonance spectroscopy (EPR) coupled with the spin trapping technique was used for determination of ?OH radicals. Results A seven-line spectrum characteristic of 5, 5-dimethyl-2-pyrrolidone-N-oxyl (DMPOX) was formed at first, at 60 min the spin adducted DMPO-OH with four-line spectrum characteristic and at 120 min, no spectrum was observed. Conclusion ?OH can be detected in AOW at 60 min and it may come from the interaction of active substance in the AOW.