Arseinc was determined through inductively coupled plasma-mass spectrometry (ICP-MS) in the acid-digested and diluted solutions of dried natural kamoku, Sargassum horneri, and its commercially processed product. The solutions from this product were obtained before and after washing/soaking with hot water, a traditional process before cooking in Japan. Each sample was also subjected to an extraction procedure for arsenic, which involved a nitric acid-based partial-digestion method by chemical speciation using HPLC-ICP-MS. The major arsenic compound detected was inorganic arsenic (V). Moreover, small amounts of organic arsenic compounds such as dimethylarsinic acid were detected, which were regarded as degradation products of dimethylated arsenosugars-the major organoarsenicals in algae.