Chlorine gas bleaching was a common method used in pulp industries. As a consequence, significant amounts of chlorine were discharged into surrounding aquatic ecosystems, affecting the biota. Chlorinated organic pollutants are formed when chlorine react with organic material. Octadecanoic acid (stearic acid) is one of the most common saturated fatty acids in aquatic biota. In a naturally occurring process two and four chlorine atoms, respectively, are added over the unsaturated bonds, forming 9,10-dichloro octadecanoic acid and 9,10,12,13-tetrachloro octadecanoic acid. These are the chlorinated fatty acids (ClFA) under investigation in this Bachelor’s Thesis. The methodological framework for measuring ClFA is investigated in this essay. The scope is to evaluate the method of isolating and quantifying the compounds as described in Åkesson-Nilsson’s (2004) dissertation. The method includes: extraction of the lipid, transesterification (where the fatty acids, including the ClFAs, are separated from the lipids and transformed into their respective methyl esters through two methods, acidic catalysis with BF3 or H2SO4), separation (by solid phase extraction) and determination of ClFA concentration with a halogen specific detector (GC-XSD/MS). Furthermore, the scope is to investigate collected fish samples (from Norrsundet) with the abovementioned method. By making a dilution series with known concentrations it was possible to establish calibration curves, to give in an indication of the effectiveness of the method. BF3 is in need of updating due to being experienced as slower and less stable than the H2SO4-method. However, it was concluded that the H2SO4-method was more effective on the standard samples and that the BF3-method was more effective on the fish lipid samples. In one of the lipid samples (lavaret transesterified with BF3) a detectable concentration of 9,10,12,13-tetrachloro octadecanoic acid was discovered. Therefore, we question SEPAs decision to cancel investigations in Norrsundet. Our results could indicate that ClFAs are still an issue that could affect the ecosystem’s biota.