This thesis is focused on studies on the environmental fate and methodological improvements for determination of the global contaminants, PCBs and DDTs, and locally discharged phenolics, e.g. 4,5,6-trichloroguaiacol (4,5,6-TCG), and their transformation products. 4,5,6-TCG is released from bleached pulp mills, and was chosen as a model substance, to study its fate and effects in aquatic environments. In a brackish water model ecosystem, 4,5,6-TCG was shown to be transformed via demethylation, dechlorination, and methylation reactions. Marine periphyton exposed to 4,5,6-TCG was shown to form demethylated, brominated and dimeric metabolites. Several of these metabolites were identified and quantified. Also more tightly bound chlorophenolics were recovered from the substrate, after harsh extraction procedures had been applied. Conjugates of chlorophenols in fish bile were determined as useful tracers for monitoring effluents from bleached pulp mills, even in areas far from the discharge point. To facilitate calculations of water concentrations, bile to water bioconcentration factors (bBCF-values) were determined for several phenolics, including also alkali-labile chlorocatechols. PCBs and DDTs, were assessed in Swedish lakes with background exposure of these global contaminants. A methodological study focus on the contamination risks of airborne PCBs, during freeze-drying and storage of dry sediment samples. Eutrophication has been proposed to cause lower levels of pollutants in biota in lakes due to biomass dilution but in this thesis examples of higher levels of PCBs in sediment traps, sediment and in perch, and also in higher fluxes of PCBs to the sediment, were observed. Phytoplankton are supposed to be responsible for most of the transport of the contaminants. The composition of DDTs in soil, sediment traps and in dated sediment cores was studied in some detail. PCBs, DDTs, HCB and HCHs were measured in sediments from 100 reference lakes included in the National Swedish Environmental Program. The lakes have a large variation in lake characteristics, representative for different areas in Sweden. A gradient was observed for sPCB with decreasing levels from the south west towards the north. The results suggests that longrange transport is the dominating mechanism for distribution of the contaminants analysed. In this study the levles of PCBs and DDTs were lower in eutrophic lakes than in oligotrophic lakes, in an area with similar atmospheric exposure. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 4: Submitted. Paper 6: Manuscript.</p>