The identification of molecular structures of an arsenolipid is pivotal for its toxicological assessment and in understanding the arsenic cycling in the environment. However, the analysis of these compounds in a lipid matrix is an ongoing challenge. So far, only a few arsenolipids have been reported, including arsenic fatty acids (AsFAs) and arsenic hydrocarbons (AsHCs). By means of RP-HPLC-ICPMS/ESMS, we investigated Capelin oil (Mallotus villosus) for possible new species of arsenolipids. Twelve arsenolipids were identified in the fish oil including three AsFAs and seven AsHCs. Among the AsHCs, four that were identified had protonotated molecular masses of 305, 331, 347, and 359 and have not been reported before. In addition, the compounds with molecular formulas CHAsO and CH AsO were found in low concentrations and showed chromatographic properties and MS data consistent with cationic trimethylarsenio fatty alcohols. Derivatization by acetylation and thiolation coupled with accurate mass spectrometry was successfully used to establish the occurrence of this new class of arsenolipids as cationic trimethylarsenio fatty alcohols (TMAsFOH).