The impact of powdered activated carbon (PAC) on the biotreatment of a chemical synthesis wastewater emerging from a pharmaceutical industry was studied. The aim of this addition was to decrease inhibitory and non-biodegradable organics. In the first step, PAC was directly added to activated sludge. In the next step, the wastewater was first contacted with PAC and then treated in an activated sludge reactor. COD, oxygen uptake rate (OUR) and ultraviolet-visible (UV-Vis) spectral measurements were carried out for the evaluation of performance. Direct PAC addition and PAC pretreatment led to similar results, but direct PAC addition was considered more practical than pretreatment with PAC. With the addition of PAC, the level of non-biodegradable matter and the colour of the wastewater were significantly reduced. In any case, the addition of PAC increased the OUR of activated sludge, indicating that inhibition could be decreased. The combined evaluation of OUR, spectral parameters and COD gave a better insight into the processes taking place than the COD parameter alone.