The formation of free hydroxyl radical (OH) in oxygen transfer reactions at PbO2 electrodes and the reaction between the electrochemically produced (OH)-O-. and salicylic acid (SA) were studied by cyclic voltammetry and capillary electrophoresis-amperometric detection (CZE-AD) in this paper. Experiments showed that (OH)-O-. was formed in the potential range of 1.0-1.4 V corresponding to the anodic discharge of H2O at the PbO2 electrode, while this phenomenon was not obvious at the Pt electrode. Optimum CZE-AD conditions for indirect determination of (OH)-O-. were explored. It was also found that when salicylic acid was used to trap (OH)-O-. produced on the PbO2 electrode surface, the amount of 2,5-dihydroxybenzoic acid produced is much more than that of 2,3-dihydroxybenzoic acid. This phenomenon is explained by the proposal that SA first forms an active aromatic radical on the PbO2 electrode surface before trapping (OH)-O-., where (OH)-O-. performs a stronger preference in attacking at the 5-position than for attacking at the 3-position of the aromatic radical. (c) 2005 Elsevier B.V. All rights reserved.