THE reduction of hydrogen peroxide by transition metal ions is thought to involve hydroxyl radicals as intermediates¹. We have found that dilute solutions of titanous ion and hydrogen peroxide (about 10^{â 2} M) react together to form a free radical of short life. This has been observed by using a flow system in which the reactants are mixed just before entering the cavity of a Varian electron spin resonance spectrometer. The spectrum is a single line near g = 2, width about 3 gauss, the intensity of which at maximum flow-rate is proportional to [Ti³⁺] and [H2O2] when these are low.