Nonylphenols are water-stable endocrine disrupting compounds that inhibit the growth of sewage bacteria in biological processes. The study describes the decomposition of 4-n-nonylphenol (NP) in water by 20 kHz ultrasound with emphasis on the impacts of pH, concentration and OH center dot scavengers. It was found that the rate of degradation was accelerated by alkalinization, but more so by the addition of hydroxide alkalinity than carbonate. The addition of low doses of CO32- and t-butyl alcohol as strong scavengers of OH center dot was also found to accelerate the decomposition of NP. The observation was attributed to the generation of reactive CO32-center dot and CH3 center dot via pyrolysis of the additives in the cavity bubbles. The results also revealed that NP did not compete with OH center dot scavenging agents when their relative concentration was low. In case of high frequency sonication (861 kHz) the competition was slightly effective (slowed down degradation) at an identical dose of t-butyl alcohol. The difference was attributed to shorter bubble life time at high frequencies leading to less violent/less energetic bubble collapse and lower yield of CH3 center dot. (C) 2009 Elsevier B.V. All rights reserved.