The effects of biogenic substrate (peptone) concentration on reactor performance, yield coefficient (Y) and degradation kinetics were evaluated in an instantaneously fed sequencing batch reactor (SBR) receiving a mixture of 4-chlorophenol (4-CP) and 2,4-dichlorophenol (2,4-DCP). High chemical oxygen demand (COD) removal efficiencies (90-95%) and complete removal of chlorophenols were observed even in the absence of peptone (i.e. chlorophenols served as sole carbon and energy sources). During the degradation of 4-CP, 5-chloro2-hydroxymuconic semialdehyde (CHMS, the meta cleavage product of 4-chlorocatechol) accumulated but was subsequently completely removed. 2,4-DCP competitively inhibited 4-CP degradation, as 4-CP degradation only commenced after complete removal of 2,4-DCP. It was concluded that decreasing peptone concentration did not affect chlorophenol degradation profiles in the reactor. The chlorophenol degradation kinetic studies suggest that competent biomass alone was responsible for chlorophenol degradation. Consequently, it was thought that competent biomass concentration was constant even though peptone concentration in the feed was varied, as competent biomass grows on chlorophenols only. Models developed using this assumption agreed well with experimental data, which also confirmed that a biogenic substrate has no impact on chlorophenol degradation profiles in an SBR with instantaneous feed. (c) 2006 Society of Chemical Industry.