Summary.Activated sludge receiving ammonium thiocyanate (500 mg/l) was able to nitrify. The rate of ammonia oxidation was decreased when>3 mg/l of phenol (or cresols) was added to the sludge, and at 10 mg/l was inhibited completely. Concentrations of up to 100 mg/l of phenols did not affect nitrite oxidation. The 2‐ and 4‐methyl pyridine derivatives inhibited both ammonia and nitrite oxidat