Ash-heap water (AHW - phenolic wastewater of local oil-shale industry) was analyzed for its phenolic composition using HPLC and for its relative toxicity using MicrotoxTM and BiotoxTM tests where inhibition of natural luminescence of photobacteria is used as toxicity endpoint. Most abundant components in AHW were phenol (84 mg/L) and p-cresol (70 mg/L). The 5-min EC50 values for AHW (the concentration of AHW, %, which reduces the light output of bacteria by 50% after being in contact with bacteria for 5 minutes) for BiotoxTM and MicrotoxTM tests were 1.5% and 0.6%, respectively. Also, the toxicity of AHW towards two different activated sludges (acclimatized and not acclimatized to phenolic wastewater) was evaluated using decrease of adenosine triphosphate (ATP) content of the sludge as toxicity endpoint. AHW was relatively non-toxic to activated sludges, especially to the acclimatized sludge. The respective 60-min EC50 value for AHW (the concentration of AHW, %, which decreases the ATP level of activated sludge compared to the non-exposed control by 50% after being in contact with sludge for 60 minutes) was 20-30% for non acclimatized sludge and 50-60% for acclimatized sludge. Therefore, the biopurification of AHW using an adapted activated sludge process could be considered feasible.