Bioaugmentation is often used to improve the removal of some recalcitrant pollutants in the contaminated water or soils. In this study, 2,4-dichlorophenol (2,4-DCP) degrading special microorganisms, which was cultured in our lab, was augmented in an activated sludge (AS) system to enhance the degradation of 2,4-DCP. The effect of a secondary chlorophenol, 4-monochlorophenol (4-MCP), which was present with 2,4-DCP, on the removal of 2,4-DCP in the bioaugmented system was investigated through batch and fed-batch tests. Results showed that the first addition of 4-MCP in feed inhibited the degradation of 2,4-DCP, and the inhibition increased with the increase of 4-MCP initial concentration. However, for the second and third fed-batch run, the system receiving the mixed components of 4-MCP and 2,4-DCP demonstrated higher 2,4-DCP removal rate than the system receiving 2,4-DCP alone. In the system with mixed components, 2,4-DCP was always degraded first, and when it nearly reached complete removal, 4-MCP degradation began. A double-stage degradation pattern was observed. (c) 2005 Elsevier Ltd. All rights reserved.