The purpose of this study was to determine the treatability of 2,4-dichlorophenol (DCP) in an anaerobic/aerobic sequential reactor system when molasses was used as carbon source. Laboratory scale upflow anaerobic sludge blanket (UASB) reactor/completely stirred tank reactor (CSTR) were operated at different 2,4-DCP loading rates and constant hydraulic retention times (HRTs) to investigate the COD removal efficiencies, accumulation of volatile fatty acid (VFA), total, methane gas productions and methane percentages. Studies were carried out in continuous mode and the effluent of the UASB reactor was used as feed of the CSTR reactor. 2,4-DCP removal efficiency decreased from 99 to 78.7% when the initial 2,4-DCP concentration and 2,4-DCP loading rates were increased from 5 to 120 mg/l and from 0.006 to 0.144 g/ (1 day), respectively. The maximum COD removal efficiency was achieved as 77% at a 2,4-DCP loading rate of 0.042 g/(1 day). When the 2,4-DCP loading rate was increased to 0.12 g/(1 day), the maximum measured VFA concentration was 1000 mg CH3COOH/l in UASB reactor. In the aerobic reactor, the COD removal efficiencies varied between 60 and 90% depending on UASB reactor treatment efficiencies and to COD concentration coming from the UASB reactor. The effects of sludge retention time (SRT) on the 2,4-DCP and COD removal efficiencies were also investigated in the aerobic reactor. The 2,4-DCP and COD removal efficiencies were observed as 90 and 85%, respectively, at a SRT of 25 days. 86.67% 2,4-DCP removal efficiency was obtained in whole sequential anaerobic/aerobic (CSTR) reactor system at an initial 2,4-DCP concentration of 120 mg/l. (c) 2005 Elsevier Ltd. All rights reserved.