The ozonation of biologically pre-treated tannery wastewater and the influence of the applied specific ozone consumption onto a subsequent biological treatment were investigated. Combining sum parameters, such as COD, DOC and UV-absorbance, it could be differentiated between mineralization and partial oxidation effects. In the case of UV-removal the newly developed partial oxidation parameters correlate with the subsequent biological removal. It could be shown that partial oxidation of COD is favourable for subsequent biodegradation, whereas further mineralization reduces the effectivity of biological oxidation. The optimal range of subsequent biological treatment was observed at a specific ozone consumption of 1 to 3 g O3/g DOC0. As far as the distribution of molecular weight fractions is concerned, the ozonation leads to a relative increase of the low molecular weight DOC-fraction (< 1,000 u), which includes the majority of the residual UV-absorbance at 254 nm. Acute toxicity, measured as luminescence inhibition of Vibrio fischeri, is affected by ozonation as well. The calculated 1/EC20 data decrease with increasing specific ozone consumption.