Nitrogen dioxide radical (NO2.) is known as a toxic agent produced in the metabolism of nitrates and nitrites. By the use of the pulse radiolysis technique, the mechanism of the reaction of NO2. radical with hydroxycinnamic acid derivatives (HCA) was studied and the rate constants have been measured. The rate constants were found to be 7.4 x 10(8), 7.2 x 10(8), 8.6 x 10(8) dm3 mol(-1) s(-1) for ferulic acid, sinapic acid and caffeic acid, respectively. The reactions produce the corresponding phenoxyl radical.