Concentrations of di, and trichlorobenzenes were determined in porewater and sediment of a sediment core from lake Ketelmeer, the Netherlands. Average in-situ partition coefficients for di, and trichlorobenzenes (log K o c ) ranged from 4.3 ± 0.2 for 1,2-dichlorobenzene to 5.0 ± 0.2 for 1,2,4-trichlorobenzene. Concentrations of di- and trichlorobenzenes in porewater were 1 to 2 orders of magnitude lower than calculated from the sediment data using literature partition coefficients. In-situ partition coefficients differed from literature values for partition coefficients obtained in lab experiments by a factor of 8 to 160. Assuming that the low concentrations in porewater can be attributed to the presence of a slow desorbing fraction it can be calculated that for most compounds studied more than 95% of the measured sediment concentration was present in the slow desorbing fraction.