Wastewater generated in industrial production processes are often contaminated by hazardous chemicals. Characterization by means of toxicity-directed analysis is useful for identifying which fractions of a waste stream possess the most toxicity. We applied this approach to evaluate toxic components of a polyester manufacturing wastewater. Using the reduction in oxygen uptake rate of activated sludge as an indicator of toxicity, it was determined that increasing the pH from 3 to 11 followed by air stripping significantly reduced the toxicity of the wastewater. Comparative headspace GC/MS analysis of wastewater at different pHs selected a group of Volatile Organic Compounds (VOCs) associated with the observed effect of air stripping at pH 11. Ten of these compounds were identified as alpha,beta unsaturated aldehydes (acrolein (2-propenal) congeners); these compounds are known to be toxic as well as mutagenic. Confirmation that these compounds were a cause of toxicity was achieved by demonstrating that removal of these compounds by air stripping significantly reduced the wastewater mutagenic potency in a Salmonella mutagenicity assay. Formation of these volatile compounds by base catalyzed aldol condensation at pH 11 may account for the effectiveness of air stripping in reducing toxicity. To date there is no record in the literature about the toxicity and presence of acrolein congeners in polyester manufacturing wastewater.61923172324Ministry of Education of Brazil