This paper describes an experimental pursuit of a mechanistic model for interpreting and quantitat ively describing nonlinear, slow, and hysteretic sorption phenomena associated with heterogeneous soils and sediment in subsurface systems.The soil/sediment organic matter was characterized using carbons types derived from ¹³C CPMAS NMR spectroscopy.All sets of equilibrium sorption data were fitted using three different sorpt ion model: the linear model, the Freundlich model or its linearized form, and the dual reactive domain model (DRDM).The linear model was not appropriate for describing any of the experimental sorption data, while the Freundlich model and the dual reactive domain model was found to be adequate for all samples.From DRDM it was observed that sorption mechanism changes with soil organic matter and with increasing contaminant concentration.The apparent sorption-desorpt ion hystersis was quantified for each sample.Sorption-desorption hystersis each varie from sample to sample, and they also appear to correlate with the chemical characteristics of soil organic matter.On the other hand, the pore structure of soils may be a critical factor in sorption-desorption hystersis of hydrophobic organic contaminants.