This paper reports on the analysis of polychlorinated biphenyls (PCBs) in selected intercomparison materials (IAEA-435 (tuna) and IAEA-159 (sediment)), and presents the author’s results against the back- ground of results accepted by the organizer of the intercomparison studies, i.e. the Marine Environment Laboratories of the International Atomic Energy Agency in Monaco. The samples were extracted with a mix- ture of acetone/hexane (tuna) or acetonitrile (sediment) by sonication, cleaned up using micro-columns packed with different materials (e.g. silica gel, florisil, copper), and analyzed by GC-ECD. The difficulties of PCB determination are indicated and the differences between the analyses of low- and high-molecular-weight PCBs are emphasized.