The main goal of this research project was to improve our understanding of the sorption processes of positively charged compounds (organic cations). Sorption to soil or sediment influences bioavailability of compounds, and it thereby affects environmental fate processes such as biodegradation, transport and toxicity. Due to strong sorption to suspended matter in river water, for example, the freely dissolved concentration may be only a small fraction of the total compound concentration in river water. Following equilibrium partitioning theory, only the freely dissolved concentration is assumed to be “bioavailable”, and aquatic toxicity may be overestimated when based on total concentrations. Likewise, it has been shown that soil toxicity of many chemicals is closely related to pore water dissolved concentrations, and not to total extractable soil concentrations. Because in most monitoring studies only total extractable concentration are reported, for risk assessment it is important to predict the freely dissolved concentrations by using an adequate sorption coefficient. This sorption coefficient is often not available and therefore relies on model predictions.