The objective of this study was to examine the reaction between ozone and p-hydroxybenzoic acid, in view of kinetic regime and by-product formation. It was found that the highest specific reduction efficiency of p-hydroxybenzoic acid, THMFP and HAAFP occurred within the kinetic regime of slow reaction, other than the very slow reaction regime. Besides, the efficient way to decrease the glyoxal formation, the ozonation by-product, is to control the reaction duration before the ozonation time of peak glyoxal, which fell within the slow reaction kinetic regime as well.