A sequential anaerobic/aerobic reactor system was used to treat a simulated textile wastewater containing azo dyes (mixing of 50 mg L-1 of Reactive Black 5, 50 mg L-1 of Direct Red 28, 50 mg L-1 of Direct Black 38, 50 mg L-1 Direct Brown 2 and 50 mg L-1 of Direct Yellow 12 as 250 mg L-1 of total concentration), soluble starch, carboxymethyl cellulose (CMC), acetic acid, glucose, salts, acids and other additives. Hydraulic retention times (HRTs) were changed to determine the effect of HRT on removal efficiencies of color, COD and total aromatic amine (TAA) through 186 days containing 46 days of steady-state and acclimation periods. COD and color removal efficiencies varying between 97% and 91 % and between 84% and 91 % were obtained at a total HRT of 19.17 and 1.22 days in combined anaerobic/aerobic system, respectively. The azo dyes decolorized under reductive anaerobic conditions in very short HRTs but the breakdown products were not ultimately metabolized and accumulated under anaerobic conditions. In the sequential aerobic stage the significant part of TAA was removed successfully while the color removal slightly increased with TAA removal efficiencies of 70-85% at total HRTs of 8.85 and 6.05 days, respectively. Increases in HRT provide enough time for partial mineralization of COD and intermetabolites in anaerobic and/or anaerobic/aerobic systems. (c) 2007 Elsevier B.V. All rights reserved.