This is a report from a project sponsored by the National Swedish Environmental Protection Board, “Environment/Cellulose”, which focuses on the chemical character, distribution pattern and potential ecological hazards of chlorinated organic substances from pulp mills. This paper deals primarily with the spread of EOCl in the sediments of the Bothnian Sea and the northern part of the Baltic. Results: – there exists a clear linkage between the spread and the concentration of EOC1 in surficial sediments and the bottom dynamic conditions, such that high contents (highest 903 µg/g ds) generally appear close to the pulp mills and within areas of accumulation, and low contents (< 1 µg/g ds) appear in sediments from areas of erosion and transportation; – diffuse leakage from land, emissions from urban areas, or from steel and petrochemical industries do not seem to be of any major importance for this characteristic distribution pattern of EOC1 in sediments; – sediment core data indicate an ongoing increase in EOCl-contamination; – the concentrations of EOC1 in sediments from lakes and coastal areas far from any known pulp mills are low; – the main conclusion from these studies is that EOC1 is distributed over vast areas and that it is hard or impossible to find any areas in the Bothnian Sea unaffected by these substances. It should be noted that in all assessments of ecological risks, it is important to have knowledge of the size and character of the impact areas, but that it may be difficult to select biological test organisms and actually establish ecological effects.