A simple and rapid technique is described for the quantitative determination of four antifouling booster biocides [diuron, 4,5-dichloro-2-n-octyl-4-isothazolin-3-one (Kathon 5287), (2-thiocyanomethylthio)benzothiazole (TCMTB) and (2,3,5,6-tetrachloro-4-methylsulphonyl) (TCMS pyridine) in aqueous samples. The analytes were extracted with high recoveries (ca. 100+/-~15%) from 2.7-l water samples, using C 1 8 solid-phase extraction. Sample extracts were quantitatively analysed by reversed-phase HPLC and polarity switching atmospheric pressure chemical ionisation (APCI) MS using selective ion monitoring. Limits of detection for the four compounds were: diuron, 1 ng/l, Kathon 5287, 1 ng/l, TCMTB, 1 ng/l and TCMS pyridine, 5 ng/l. Analysis of samples collected from various UK marinas showed detectable concentrations of diuron to be present, however, concentrations of other three booster biocides were below their respective limit of detection.