The generation of hydroxyl radicals ((OH)-O-center dot) during the chlorination of air saturated solutions of different hydroxyphenols (hydroquinone, resorcinol, catechol, gallic and tannic acids) at pH 7 has been determined by the formation of phenol (in presence of benzene in excess) or 2-hydroxyterephthalic acid (in presence of terephthalic acid). Formation of (OH)-O-center dot was only detected during the chlorination of o- or p-hydroxyphenols, compounds that react with chlorine by electron transfer forming the corresponding semiquinones/quinones. In aerated solutions, oxygen is reduced by the semiquinone to the superoxide radical, O-2(center dot-), which reacts with HOCl to (OH)-O-center dot. Compared to the studied o-hydroxyphenols, the lower reactivity of hydroquinone towards chlorine favours the reaction between chlorine and O-2(center dot-), and its (OH)-O-center dot formation potential is -similar to 50 times higher. The extent of (OH)-O-center dot generated increased with the concentration of the hydroxyphenol and chlorine, but the (OH)-O-center dot yield (moles formed per mole of hydroxyphenol eliminated), decreased due to the formation of the quinone, that acts as O-2(center dot-) scavenger. The yield was almost not affected by the pH (6 <= pH <= 7.5), whereas a strong impact of dissolved O-2 was observed. The (OH)-O-center dot production was null in absence of O-2 and 2.5-3 times higher at oxygen saturated conditions compared to air-saturated. Contrary to chlorination, during bromination of hydroquinone (OH)-O-center dot was not formed, which can be attributable to a much faster consumption of the oxidant, with no chance for O-2(center dot-) to react with bromine. Formation of (OH)-O-center dot during the chlorination of different NOM extracts (SRHA, SRFA, PLFA and Nordic Lake NOM) and water from Lake Greifensee (Switzerland) was also studied using terephthalic acid as (OH)-O-center dot scavenger. For SRHA, SRFA and Nordic Lake NOM (all of allochthonous origin and presenting high electron-donating capacity, EDC), (OH)-O-center dot yields expressed as moles formed per mole of DOC0 (%), were between 1.1 and 2.0, similar to that of hydroquinone (similar to 1.5). For PLFA and Lake Greifensee water (autochthonous, lower EDC) much lower (OH)-O-center dot yields were observed (0.1-0.3). Both chlorination rate and EDC, the later favouring the formation/stabilization of O-2(center dot-), seem to be key factors involved in (OH)-O-center dot generation during the chlorination of NOM. A mechanism for these findings is proposed based on kinetic simulations of hydroquinone chlorination at pH 7. (C) 2020 The Authors. Published by Elsevier Ltd.