Salicylic acid (SA, hydroxybenzoic acid) was used as molecular probe to determine hydroxyl radical ((OH)-O-center dot) in aqueous solutions, where microwave (MW) radiation was involved and activated carbon (AC) existed. Rapid reaction of (OH)-O-center dot with SA produced a stable fluorescence product, 2,3-dihydroxybenzoic acid (2,3-DHBA) as well as its homologue, 2,5-DHBA, which were determined by HPLC. Dose of AC, air supply, and MW power proved to be key factors influencing (OH)-O-center dot generation under MW radiation. The rate of (OH)-O-center dot generation reached 0.036 mu mol s(-1), and the (OH)-O-center dot yield reached 3.2 mu mol in 3 min under a given condition. (OH)-O-center dot is an active species with high oxidative activity and can initiate free radical reactions. Another experiment was carried out to confirm that organic substance may degrade in aqueous solution by the MW assisted catalytic process. Pentachlorophenol (PCP) was used as target. Results showed that nearly 72-100% degradation (corresponding to 40% and 82% TOC removal) was observed in 60 min for two levels of PCP concentrations (500 and 2000 mg L-1), which implies that continuous production of (OH)-O-center dot during MW radiation supports free radical reactions that benefit PCP degradation. (c) 2006 Elsevier B.V. All rights reserved.