Ozonation of a natural tannin (NT; COD0 = 1195 mg/ L; TOC0 = 342 mg/L; BOD5.0 = 86 mg/L) and a synthetic tannin ST; COD0 = 465 mg/L; TOC0 = 55 mg/L; BOD5.0 = 6 mg/L) being frequently applied in the polyamide dyeing process was investigated. Synthetic wastewater samples containing these tannins individually were prepared and subjected to ozonation at varying ozone doses (625-1250 mgO(3)/L wastewater), at pH =3.5 (the application pH of tannins) and pH = 7.0 at an ozone dose of 1125 mgO(3)/L wastewater. The collective environmental parameters COD, TOC, BOD5, UV254 and UV280 (UV absorbance at 254 not and 280 nm, representing aromatic and unsaturated moieties, respectively) were followed during ozonation. Changes in the biodegradability of the tannins were evaluated in terms of BOD5 measurements conducted before and after ozonation. In addition, activated sludge inhibition tests employing heterotrophic biomass were run to elucidate the inhibitory effect of raw and ozonated textile tannins towards activated sludge biomass. Partial oxidation (45% COD removal at an ozone dose of 750 mg O-3/L wastewater and pH = 3.5) of ST was sufficient to achieve elimination of its inhibitory effect towards heterotrophic biomass and acceptable biodegradability improvement, whereas the inhibitory effect and biodegradability of NT could not be reduced via ozonation under the same reaction conditions.