The metabolic capability of denitrifying sludge to oxidize ammonium and p-cresol was evaluated in batch cultures. Ammonium oxidation was studied in presence of nitrite and/or p-cresol by 55 h. At 50 mg/L NH(4)(+)-N and 76 mg/L NO(2)(-)-N, the substrates were consumed at 100% and 95%, respectively, being N(2) the product. At 50 mg/L NH(4)(+)-N and 133 mg/L NO(2)(-)-N, the consumption efficiencies decreased to 96% and 70%, respectively. The increase in nitrite concentration affected the ammonium oxidation rate. Nonetheless, the N(2) production rate did not change. In organotrophic denitrification, the p-cresol oxidation rate was slower than ammonium oxidation. In litho-organotrophic cultures, the p-cresol and ammonium oxidation rates were affected at 133 mg/L NO(2)(-)-N. Nonetheless, at 76 mg/L NO(2)(-)-N the denitrifying sludge oxidized ammonium and p-cresol, but at different rate. Finally, this is the first work reporting the simultaneous oxidation of ammonium and p-cresol with the production of N(2) from denitrifying sludge. (C) 2011 Elsevier Ltd. All rights reserved.