The kinetic behavior of a nitrifying sludge exposed to 2-chlorophenol (2-CP) was evaluated in batch culture. The assays were performed using a stabilized nitrifying sludge. In control assays with (mg L-1): NH4+-N (100) and NaHCO3--C (250), the substrates were consumed in 8h. the ammonium consumption efficiency was 99% and the NO3- yield higher than 0.9. When 5 mg 2-CP-C L-1 was added, it was transformed into an unidentified intermediate and the nitrifying efficiency decreased to 10%. Ammonium specific consumption rate diminished 95%, but the NO3-yield remained higher than 0.9. The biomass previously exposed to 2-CP was newly suspended with NH4+-N or NO2--N in order to evaluate the ammonium and nitrite oxidizing processes. The consumption efficiencies and NO3- yields were similar to those obtained in control assays. However, the total time required for ammonium and nitrite consumption increased to 120 and 42h. respectively. Specific consumption rates for NH4+-N and NO2--N decreased by 95% and 83% respectively, compared to control assays. Thus, the previous contact to 2-CP had more influence on ammonium oxidizing process than the nitrite oxidizing process. These are the first evidences where a nitrifying sludge exposed to 2-CP are reported. (C) 2010 Elsevier B.V. All rights reserved.