The effects of the addition of a secondary carbon source on the biodegradation of a chlorinated phenol were studied with a series of batch reactors. The biodegradability of chlorinated phenols was shown to follow the decreasing order: phenol > 2,4-DCP > 4CP > 2,4,6-TCP > 2CP > 3CP by the unacclimated microorganisms. The addition of a secondary carbon source may enhance or retard the biodegradation of a recalcitrant compound. The presence of a relatively easily biodegraded compound, such as 2,4-dichlorophenol, enhanced the biodegradation of a less chlorinated but recalcitrant compound, such as 2-chlorophenol. The enhancement of the removal efficiency of 2-chlorophenol was proportional to the concentration of 2,4-dichlorophenol. Monochlorophenols, such as 2-chlorophenol, 3-chlorophenol or 4-chlorophenol generally retarded the biodegradation of the relatively more highly chlorinated phenols, such as 2,4-dichlorophenol or 2,4,6-trichlorophenol.