Effect of sludge and initial dyestuff concentration on color and COD removal performance of anaerobic-aerobic sequential batch reactor was investigated. Remazol Red RR a vinylsulphonyl (VS) and monochlortriazine (MCT), reactive azo dye was used in the study. Sludge age was varied between theta(C) = 12 days and theta(C) = 30 days and dyestuff concentration was between D-0 = 50 and D-0 = 500 mg l(-1). The maximum color and COD removal was obtained as 95% and 70% for D-0 = 60 mg l(-1) and COD0 = 800 mg l(-1) at 15 days sludge retention time, respectively, and no further improvement was observed when sludge age was increased to 30 days. The main color removal phase in this operation system was the anaerobic phase. Because, the color removal efficiency was already above 95% under anaerobic condition and therefore, the contribution of aerobic phase to color removal was negligible. Increasing dyestuff concentration did not significantly affect the decolorization. It was possible to obtain over 90% dyestuff removal even at D-0 = 500 mg l(-1). SBR system reduces 1000 mg l(-1) initial COD concentrations to about 400 mg l(-1) for dyestuff concentration up to 150 mg l(-1). COD removal efficiency decreased from 70% to 60% by increasing initial dyestuff concentration from 100 to 500 mg l(-1). The results indicated that dyestuff and COD are mainly used by anaerobic organisms and aeration does not improve the performance of SBR system. (c) 2005 Elsevier B.V. All rights reserved.