In this study, the optimum operating conditions of a lab-scale completely stirred acidogenic reactor treating a chemical synthesis-based pharmaceutical wastewater were determined. Individual effects of hydraulic retention time (HRT), organic loading rate (OLR) and pH on the degree of acidification were investigated at a mesophilic temperature (35 +/- 1 degrees C), for HRT in the range 8-24 h, OLR up to 14 kg COD/m(3) d and pH in the range of 5.0-6.3. A maximum acidification level of 44% and COD removal efficiency of 13%, were obtained at an OLR of 13 kg COD/m(3) d at pH of 5.5 +/- 0.1 with an HRT of 12 It. Under these conditions, acetic, propionic and n-butyric acids were found to be predominant volatile fatty acids in the acidogenic reactor. (c) 2006 Elsevier Ltd. All rights reserved.