Arsenic determination comprised methods based on hydride-generation atomic absorption spectrometry (HG-AAS). Conventional HG-AAS allows the determination of arsenic compounds which form volatile hydrides on reduction. Speciation of these compounds was achieved using HPLC (anion-exchange and ion-pair) and cryogenic trap methods coupled to the HG-AAS manifold. Optimum detection limits were achieved for the cryogenic trap system, with detection limits in the picogram range. The anion-exchange HPLC method was enhanced to respond to compounds which have not previously been detected using such methods by the incorporation of a photooxidation step placed on-line, between the HPLC column and the HG manifold. Detection limits are ca 0.3 μg As/l. Batch ultraviolet irradiation was also employed to photooxidise arsenic compounds that are not reduced to volatile hydrides. Arsenobetaine and arsenocholine are not detected using HG-AAS alone, but after overnight ultraviolet irradiation, they were photooxidised to a mixture of inorganic arsenic and mono-, di- and tri-methylarsenic, and hence could be detected using conventional HG-AAS methods. A survey of the Carnon River, Cornwall, showed the presence of high levels of arsenite in the river (240 μg As/l) and estuary (27 μg As/l) when measured a few weeks after an acute discharge of mine leachate into the river. Five months after the discharge, arsenite levels in the river had fallen to below detection limits, but estuarine levels were still elevated (ca 12 μg As/l). The release of arsenic compounds by marine organisms was studied using the alga Pelvetia canaliculata. Under laboratory conditions, the alga released high levels of arsenosugars to the surrounding water, particularly when placed under low salinity conditions. When kept in tanks of artificial seawater, the alga released inorganic arsenic, mono- and dimethyl arsenic, and arsenic species which required photoxidation in order to be detected into the water. Thus, Pelvetia canaliculata, which is found in many estuaries and coastal waters, is likely to be a source of several arsenic compounds to water in these regions.