-Arsenic content in dominant seaweed from the Thermaikos Gulf was determined -Total arsenic values are lower than those found in other Mediterranean seaweeds -As speciation of water extracts from the algae are performed by LC-ICP-MS -Arsenosugars are measured in all samples The content of total arsenic and arsenic compounds in the dominant seaweed species in the 18 Thermaikos Gulf, Northern Aegean Sea, was determined in samples collected in different seasons.