Concentration of hydroxyl radicals generated from ozone microbubbles was determined by using an indirect method over a wide range of pH (i.e. pH 3-10). p-Chlorobenzoic acid (PCBA) was used as the probe compound to calculate the concentration of hydroxyl radicals. A constant, R-ct, was defined as the ratio of (OH)-O-center dot and O-3 exposures, which was employed to estimate the concentration of hydroxyl radicals. The effects of pH and carbonate ions on the generation of hydroxyl radicals were studied. It was observed that the O-3 exposure and the depletion of PCBA were higher at acidic pH than at the alkaline pH. At acidic pH, CO32- acted as the center dot OH scavenger only, whereas at alkaline pH, it inhibited ozone decomposition. This changed the values of center dot OH exposure, O-3 exposure, and hence, R-ct. The degradation of phenol was performed at pH 3-10 by using ozone microbubbles. The contributions of center dot OH and O-3 on the oxidation of phenol were calculated. A mechanism of the generation of center dot OH from the ozone microbubbles under acidic and alkaline conditions was developed. (c) 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.