In this study, the efficiency of the integrated process was investigated in which the pre-ozonation step was followed by activated sludge process (ASP) in treating the aqueous Acid Red-151 (AR 151) solutions. The percent dye removal in the integrated process was found to be 47% for a pre-ozonation time of 30 min, instead of 25% in the singly ASP without pre-ozonation. The treatment efficiency of this process could be higher if the pre-ozonation time of 120 min yielding maximum enhancement of biodegradability at a peak value of BOD5/COD ratio were used before the biological treatment process. (c) 2006 Elsevier B.V. All rights reserved.