Phenolic compounds biodegradation and its effect on the nitrification process were studied. A continuous stirrer tank reactor was operated in four stages, and phenolic compounds were fed as sequential way. In the first stage, at loading rate of 220 mg NH4+-N/L d, the consumption efficiency was of 91%, being the product, nitrate. After that, p-cresol was fed at 53 mg C/L d, reaching removal efficiencies for both substrates higher than 90%. In the third stage, p-hydroxybenzoate was fed at 56 mg C/L d, and the removal efficiencies for all substrates remained high. In the last stage, the reactor was fed at 54 mg C/L d of phenol, and it caused a diminishing on the ammonium removal efficiency; however, all phenolic compounds were efficiently removed. Kinetic results showed that the presence of each phenolic compound improved the ammonium oxidizing activity, but the nitrite oxidizing activity was not affected. (C) 2012 Elsevier Ltd. All rights reserved.