A method for the determination of arsenate, arsenite, monomethylarsonate and dimethylarsinate was developed using hydride generation and cold trapping of the hydrides, coupled with atomic fluorescence detection at 193.7 nm. Optimisation of the signal was carried out for a number of experimental parameters, including the carrier gas flow rates, sample volume and the concentration and addition time of NaBH 4 . The detection limits for As III , As V , MMA and DMA are 2.3, 0.9, 2.4 and 3.7 ng l –1 , respectively, in a 5 ml sample. The precision for nine sample replicates was better than 3.5% for all the arsenic species. The accuracy of the method was determined by analysis of a sea-water reference material (NASS-4) and by recovery studies on natural samples. Examples of the method's application to the analysis of arsenic species in unpolluted coastal sea-water are given.