The ozone degradation of 2,4-dichlorophenol (2,4-DCP), as toxic model pollutant in wastewater, is investigated in the presence of readily biodegradable substrates. Since a successful treatment does not only depend on the elimination of the toxic compound, a proper assessment of the residual toxicity and/or biodegradability of the ozonated wastewater is necessary. Respiromety is used to evaluate the biodegradation efficiency and microbial activity of both non-acclimated and pre-exposed biomass. Ozone pre-treatment results in a decrease in 2,4-DCP and an increase in chloride concentration. The release of chloride is inversely proportional to the degradation of 2,4-DCP, implying the dechlorination of the aromatic ring as a first reaction step in its decomposition. Furthermore, the degradation of 2,4-DCP is described by pseudo first-order kinetics. The degradation rate decreases when biodegradable organic matter is present in the wastewater and the extent of this decrease depends on the nature of the biodegradable organic matter. The measured COD degradation does not correspond with the mineralization of 2,4-DCP. Hence, the pollutant is instead degraded into dehalogenated intermediates. Further, respirometric measurements are performed in order to investigate the effects on the toxicity of the wastewater. The period before biomass activity is observed, the so-called lag phase, is significantly reduced and, furthermore, the maximal Specific Oxygen Uptake Rate (SOUR), indicating biomass activity, increases with increasing ozone reaction time. This means that the observed toxic effects are reduced. Also, the biodegradable fraction increases and the amount of refractory COD decreases by the ozone treatment because of the (partial) degradation of the pollutant. Therefore, the biodegradation can be performed in a shorter period of time. (C) 2015 Elsevier B.V. All rights reserved.