The competitive kinetics technique is shown to be a useful and reliable tool for determining rate constants. Regardless of the conditions of the reaction and the operation mode, the intrinsic second-order rate constants of 2,6-dimethyl-aniline and hydroxyl radicals were 1.65 x 10(10) 1.60 x 10(10), and 1.71 x 10(10) M-1 s(-1) in the absence of SiO2 under complete-mix conditions, in the presence of SiO2 under complete-mix conditions, and in a fluidized-bed Fenton reactor with SiO2 as the media, respectively, demonstrating that the rates are comparable under a variety of reaction conditions. The average intrinsic second-order rate constant of o-toluidine and hydroxyl radicals obtained in a homogeneous system under various conditions was 7.36 x 10(9) M-1 s(-1), indicating that o-toluidine is less susceptible to hydroxyl radicals than 2,6-dimethyl-anilne. Hydroxyl radicals primarily attacked the amine group rather than the methyl group of the o-toluidine to form o-cresol and 2-nitrotoluene, which sequentially transformed to carboxylic acids including acetic, oxalic, lactic, and maleic acids after the collapse of the benzene ring. (C) 2011 Elsevier B.V. All rights reserved.