Ultrasonic degradation of 2,4-dichlorophenol (2,4-DCP) has been studied under oxygen, air, argon, and nitrogen in aqueous solutions fora liquid temperature of 20 degrees C at 489 kHz. The degradation rate increases significantly in the presence of O-2, and argon, whereas it remains the lowest under nitrogen, for which a competitive reaction model has been proposed for its non-exponential decay. Experiments have been also performed at three different pHs, 11.0, 6.3, and 2.0. The significant degradation has been achieved at pH 6.3 under O-2(0.86 x 10(-3) s(-1)), which is 1.9 and 4 times higher than acidic (pH 2.0), and basic (pH 11.0) conditions, respectively. The degradation rates have decreased in the order O-2 > Ar > air > N-2 irrespective of pH. (C) 2009 Elsevier B.V. All rights reserved.