Two major antifouling biocides used worldwide, Irgarol 1051 and diuron, and their degradation products in Shoreham Harbour and Brighton Marina, UK were studied during 2003–2004. The highest concentrations of Irgarol 1051 were 136 and 102 ng L−1 in water and 40 and 49 ng g−1 dry weight in sediments for Shoreham Harbour and Brighton Marina, respectively. As the degradation product of Irgarol 1051, M1 was also widespread, with the highest concentration of 59 ng L−1 in water and 23 ng g−1 in sediments in Shoreham Harbour, and 37 ng L−1 in water and 5.6 ng g−1 in sediments in Brighton Marina. The target compounds showed enhanced concentrations during the boating season (May–July), when boats were being re-painted (January–February), and where the density of pleasure crafts was high. Overall, the concentration of Irgarol 1051 decreased significantly from late 2000 to early 2004, indicating the effectiveness of controlling its concentrations in the marine environment following restricted use. Diuron was only detected in 14% of water samples, and mostly absent from sediment samples. © 2006 Elsevier Ltd. All rights reserved.