Arsenic is commonly known as poison and contaminant from both natural and anthropogenic sources. However, it is mostly unknown to the general public that there is a wide variety of arsenic species, which can either be toxic or benign. Currently, public interest and legislation focuses on inorganic species of arsenic. However, there is increasing evidence for the toxic capabilities of organic arsenic containing compounds, especially arsenolipids. These compounds are commonly found in oils derived from marine animals, such as fish oil or krill oil, which are increasingly popular as food supplements due to their high omega-3 fatty acid content. Other samples of (mostly) marine origin have also been shown to contain arsenolipids. In this thesis, organ samples from a stranded pod of long-finned pilot whales (Globicephala melas) were investigated for their arsenolipid content. This was achieved by coupling reversed phase high performance liquid chromatography (RP-HPLC) with a combination of inductively coupled plasma mass spectrometry (ICP-MS) and electrospray ionisation mass spectrometry (ESI-MS). Using these techniques, it is shown that arsenic can accumulate with age in mammalian brains in the form of arsenic containing phospholipids (AsPL). In contrast to inorganic arsenic and due to their lipophilic nature, lipophilic arsenic containing compounds are thus able to pass the blood-brain barrier. In a second part of the thesis, a previously unknown type of lipophilic arsenic containing compounds has been identified in strains of Streptomyces bacteria. This compound was shown to feature a non-methylated arsenic moiety, which is a structural feature that has not been reported for any of the previously known arsenolipids.