The effect of ozonation on the biodegradability and acute toxicity of two frequently used textile dye assisting chemicals, namely natural tannin (NT) having an initial chemical oxygen demand (COD0) of 1195 mg L-1 ; initial total organic carbon (TOC0) of 342 mg L-1 ; initial 5th-day biochemical oxygen demand (BOD5,0) of 86 mg L-1 ; initial ultraviolet absorbance at 280 nm wavelength (UV280,0) of 32.2 cm(-1) ; initial ultraviolet absorbance at 254 nm wavelength (UV254,0) of 19.35 cm(-1) and synthetic tannin (ST); with a COD0 of 465 mg L-1 ; TOC0 of 155 mg L-1 ; BOD5,0 of 6 mg L-1 ; UV280,0 of 11.78 cm(-1) ; UV254,0 of 13.74 cm(-1) was investigated. Synthetic wastewater bearing these tannin formulations was individually prepared and subjected to ozonation at varying doses (500 and 1000 mg h(-1) ), and pHs (3.5 and 7.0) to elucidate the effect of ozone dose and pH on oxidation efficiency. Changes in the environmental sum parameters chemical oxygen demand (COD), total organic carbon (TOC), 5th-day biochemical oxygen demand (BOD5), ultraviolet absorbance at 280 nm wavelength (UV280) and ultraviolet absorbance at 254 nm wavelength (UV254), the latter two representing the aromaticity and double bonds of the studied textile tannins, respectively, were monitored during the course of ozonation. In the second part of the study, the biodegradability and acute toxicity of the raw and pre-ozonated textile tannins were evaluated in terms of the BOD5 parameter and an activated sludge inhibition test, respectively. Results indicated no significant changes in acute toxicity for NT, whereas the inhibitory effect of ST could be completely eliminated after 40 min ozonation at a rate of 1000 mg h(-1) (i.e., at a specific ozone dose of 1.4 mg O-3 (mg COD0)(-1) and a pH of 3.5. In conclusion, pre-ozonation appeared to be potential pretreatment option to achieve complete detoxification and a fair biodegradability improvement of the otherwise refractory synthetic tannin.