Surface water samples were collected during February 2001 along 11 transects and at four stations in the Western Baltic Sea and the Baltic Proper and analysed for polychlorinated biphenyls (PCBs) and hexachlorobenzene (HCB). The agreement between PCB concentrations in parallel transect samples averaged f 20%, indicating good method reproducibility. The sampling system separated the organic contaminants into a particle-associated fraction and a dissolved fraction. The dissolved concentrations were very homogenous in the Baltic Proper, the relative standard error of the mean being < 10%. Beginning in the Arkona Sea, the dissolved PCB concentrations increased moving westward, suggesting the presence of sources in the Danish Straits or higher PCB concentrations in inflowing North Sea water. The PCB fugacities in the surface water of the Baltic Proper were similar to fugacities measured in winter air in the region, indicating that PCBs in the surface water and the atmosphere were close to a partitioning equilibrium. The particle-associated concentrations varied with both the particulate organic carbon (POC) concentration and the PCB/HCB concentrations in solution. The log organic carbonwater partition coefficients (K OC ) of the contaminants were very similar in all samples, with a standard deviation of 0.1 -0.15 log units. Between chemicals an excellent linear correlation between log K OC and the log octanol -water partition coefficient (log K OW ) was obtained (r 2 = 0.98). However, the slope of 0.78 suggests that octanol is not a good model for the sorption properties of the POC. This data set provides a sound basis for the interpretation of PCB and HCB fate in the Baltic Sea.