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−1 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 high‐pressure nebulizer served as the arsenic‐specific detector. Arsenobetaine (0.052–1.67 mg As kg−1 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−1 wet mass). The tetramethylarsonium cation was detected in all pinnipeds (<0.009 to 0.043 mg As kg−1) but not in any of the cetaceans. The concentration of dimethylarsinic acid ranged from < 0.001 to 0.109 mg As kg−1 wet mass. Most of the concentrations for methylarsonic acid (<0.001 to 0.025 mg As kg−1 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−1). An unknown arsenic compound was present in all liver samples at concentrations from 0.002–0.027 mg As kg−1. © 1998 John Wiley & Sons, Ltd.