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Sorption behaviour of pentachlorobenzene on four humic acids (HAs) and two humins extracted from a take sediment was studied. The objective was to provide a better understanding of the sorption mechanism of pentachlorobenzene onto sediment organic matter. Ten HAs were obtained by progressive sequential extraction of a sediment sample, while the residue after extraction was separated into two humin fractions on the basis of organic carbon content. The structural heterogeneity of the HAs and humins was characterized using elemental analysis and diffuse reflectance Fourier transform infrared spectroscopy. All sorption isotherms, fitted to a Freundlich equation, were nonlinear. For the HAs, isotherm linearity increased from 0.902 for the first extracted HA to 0.945 for the tenth HA, showing increase in the nonlinearity of the associated sorption isotherms with increasing HA aromaticity. On the other hand, a positive correlation between sorption coefficient and aliphaticity of the HAs supports the hypothesis that the non-polar aliphatic domains in sediment organic matter strongly affect the sorption of hydrophobic organic compounds. Both humin samples showed greater nonlinearity than the HAs, with the greatest degree of nonlinearity (0.827) for the humin with lower organic carbon content, while humin with higher organic carbon content showed the greatest pentachlorobenzene sorption affinity. It has been suggested that sorption processes may be strongly affected by the physical conformation of, and accessibility to, organic matter, as demonstrated by the high K-oc and low n of humin samples. (c) 2007 Elsevier Ltd. All rights reserved.