The effect of powdered activated carbon (PAC) on the biotreatment of an industrial wastewater taken from a chemical plant synthesising drugs for the pharmaceutical sector was studied. Industrial and domestic wastewaters were combined at laboratory scale and the effect of PAC addition was tested in aerobic reactors. The aim of this addition was to decrease inhibitory and nonbiodegradable organics. Two different procedures were applied in testing the effectiveness of PAC. First, PAC was directly added to activated sludge mixed liquor. In the second case, industrial wastewater was first contacted with PAC and then treated biologically. In the evaluation of performance, COD measurements, oxygen uptake rate (OUR) measurements and measurements in the ultraviolet-visible (UV-Vis) spectra were taken into consideration. Both direct PAC addition and PAC pretreatment led finally to similar results. In both cases, the concentration of non-biodegradable matter could be lowered. In particular, the colour of the wastewater was significantly reduced. Direct PAC addition appeared to be more practical and plausible. In any case, PAC addition to activated sludge increased the OUR of the sludge, indicating that inhibition could be decreased. The study also pointed out that in the assessment of PAC performance, the combined evaluation of OUR, spectral parameters and COD would be much more informative than the COD parameter alone.