Arsenic is present in seawater in trace amounts (ca 1 g/kg, 13 nM) mostly as the oxoanion arsenate. In contrast, arsenic is present in marine biota (animals and algae) at very high levels (up to 100,000 g/Kg. In marine animals, arsenobetaine (AB, Me3As+CH2COO-) is the major arsenic species while in algae arsenosugars (various dimethylarsinoylriboside derivatives) are the predominant species detected; smaller amounts of arsenolipids are found in both animals and algae. These compounds have never been detected in seawater even though they must be excreted to the seawater at some stage. The complex nature of seawater (3.5 % salt) presents an analytical challenge to detect the compounds and monitor their environmental path. In a first step we developed a system that comprised a sample preparation method using preparative cation-exchange media combined with high performance liquid chromatography coupled to an inductively coupled plasma mass spectrometry (HPLC/ICPMS) and high performance liquid chromatography coupled to an electrospray mass spectrometer HPLC/ESIMS to study the fate of organoarsenicals under conditions mimicking the natural environment. These methods remove >99 % of the complex sample matrix while recovering > 80 % of the target arsenic compounds. The method was applied to experimental systems investigating the stability of these arsenic compounds in seawater at environmentally relevant concentrations under natural conditions. In a second step we developed a sample preparation method using liquid-liquid extraction combined with HPLC/ESI-Orbitrap-mass spectrometry for detecting arsenolipids in seawater. This method removed > 99 % of the complex sample matrix while recovering > 90 % of the target arsenic compounds. The method was applied to seawater for the detection of arsenolipids. None of the arsenolipids was detected in seawater but the method can be used to investigate the stability of these arsenic compounds in seawater at environmentally relevant concentrations under natural conditions. In a third step we investigated the stability of three arsenolipids, common constituents of fish and algae, relevant to sample storage and transport, and to their preparation for quantitative measurements. The compounds investigated were two arsenic fatty acids (a saturated example, AsFA-362, and the unsaturated AsFA-388) and one saturated arsenic hydrocarbon (AsHC-332); the fate of the arsenolipids was followed by HPLC/MS/MS analyses. Storage of the compounds, dry as pure compounds or mixed in fish oil, at up to 60C did not result in significant changes to the compounds, although losses were observed by apparent adsorption onto the plastic walls of the polypropylene tubes. No losses occurred when the experiment was repeated with glass tubes. When the compounds were stored in ethanol for up to 15 days under acidic, neutral, or alkaline conditions (each at room temperature), no significant decomposition was observed, although esterification of the fatty acids occurred at low pH. The compounds were also stable during a sample preparation step involving passage through a small silica column. The results indicate that these typical arsenolipids are stable when stored in glass at temperatures up to 60C for at least 2 days, and that; consequently, samples of food or extracts thereof can be transported dry at ambient temperatures, i.e. without the need for cool conditions.