Total arsenic concentrations and the concentrations of individual arsenic compounds were determined in liver samples of pinnipeds [nine ringed seals (Phoca hispida), one bearded seal (Erginathus barbatus)] and cetaceans [two pilot whales (Globicephalus melas), one beluga whale (Deliphinapterus leucus)]. Total arsenic concentrations ranged from 0.167 to 2.40 mg As kg ร1 wet mass. The arsenic compounds extracted from the liver samples with a methanol/water mixture (9:1, v/v) were identified and quantified by anion-and cation-exchange chromatography. An ICP-MS equipped with a hydraulic highpressure nebulizer served as the arsenic-specific detector. Arsenobetaine (0.052-1.67 mg As kg ร1 wet mass) was the predominant arsenic compound in all the liver samples. Arsenocholine was present in all livers (0.005-0.044 mg As kg ร1 wet mass). The tetramethylarsonium cation was detected in all pinnipeds (0.009 to 0.043 mg As kg ร1 ) but not in any of the cetaceans. The concentration of dimethylarsinic acid ranged from 0.001 to 0.109 mg As kg ร1 wet mass. Most of the concentrations for methylarsonic acid (`0.001 to 0.025 mg As kg ร1 wet mass) were below the detection limit. Arsenous acid and arsenic acid concentrations were below the detection limit of the method (0.001 mg As kg ร1 ). An unknown arsenic compound was present in all liver samples at concentrations from 0.002-0.027 mg As kg ร1 .