A method based on high performance liquid chromatography (HPLC), has been developed to measure hydroxyl radical ((center dot)OH) in plasma reactors. The determination was performed indirectly by detecting the products of the reaction of (center dot)OH with salicylic acid (SAL). The applicability, and effect of time, specific input energy (SIE), relative humidity (RH), catalyst were investigated. It was found that 3 h was the optimal trapping time; concentration of (center dot)OH was (5.9-23.6) x 10(13) radicals/cm(3) at SIE range. The highest (center dot)OH yield and toluene removal efficiency (eta) were achieved with a RH of 20%. With MnO (x) , eta was two times that without catalyst, while (center dot)OH yield in gas stream was one-sixth that without catalyst. However, if summed with (center dot)OH adsorbed on catalyst surface, the total (center dot)OH yield was the same as without catalyst. Experiments performed with/without toluene allowed to determine the role of (center dot)OH on decomposition of toluene in air plasma.