The treatability of Remazol Black-5 was studied in an anaerobic/aerobic sequential process. Laboratory scale UASB/CSTR reactors were used and operated at different organic loadings and HRTs to investigate the COD, color removal, and methane gas production efficiencies. The effect of both sludge age and HRT on the color and COD removal efficiencies were also investigated. The reactive dye used in this study contains the groups N = N, −SO3, S = O and is in use in some of the textile industries in Turkey. The studies were carried out in continuous mode and the effluent of the UASB reactor was used as feed in the CSTR reactor. COD removal efficiencies decreased from 56 to 27% with increases in COD loadings from 5 kg COD/m3.day and 25 kg COD/m3.day in the anaerobic UASB reactor. The color removals were 92 and 87%, respectively, for the aforementioned organic loadings. The methane percentages were found to be 50 and 76% at organic loading rates of 2.49 kg COD/m3.day and 14.8 kg COD/m3.day, respectively. 28, 42, and 90% COD removal efficiencies were obtained at sludge retention times of 1.7, 5.7 and 11 days in the aerobic CSTR reactor. Optimum sludge age was 11 days in aerobic reactor and 67 and 28% COD removal efficiencies were obtained at F/M ratios of 0.05–0.17 and 0.30–1.4 kg COD/kg MLSS.day. 90–95% color and 40–60% COD removal efficiencies were obtained depending on the applied organic loadings in the UASB reactor. The remaining COD was removed with a treatment efficiency of 85–90% in the aerobic CSTR reactor.