Within the framework of the Standards, Measurements and Testing Programme of the European Commission, a certification campaign was conducted for two freshwater sediment reference materials for polycyclic aromatic hydrocarbons and polychlorinated biphenyls, respectively. The sediments were collected from a Dutch river, dried, ground, homogenized, packed and tested on stability and homogeneity. The materials were then analyzed by a group of laboratories with proven competence in the field. The contents of seven PAHs (pyrene, benz(a)anthracene, benzo(a)pyrene, benzo(e)pyrene, benzo(b)fluoranthene, benzo(k)fluoranthene and indeno(1,2,3-cd)pyrene) were certified in one material (CRM 535), and the contents of 13 PCBs (IUPAC Nos. 28, 52, 101, 105, 118, 128, 138, 149, 153, 156, 163, 170 and 180) in the other (CRM 536). The materials are available from the Community Bureau of Reference of the European Commission (BCR) to all laboratories that want to validate their PAH or PCB methods for trueness. Copyright (C) 1999 Elsevier Science B.V.