The partition coefficients (K ow) of benzoic acid and sorbic acid in systems of fish oil (sand eel)-water, fish oil-buffer solution, rape oil-water and olive oil-water were experimentally determined in a temperature range from 5 to 43°C and pH from 4.5 to 6.5°C. The dimerization of benzoic acid in fish oil-water system was observed at 25°C. Two modifications have been made to the Nordic Food Analysis Standard for the determination of sorbic acid by HPLC. The experimental results show that the K ow of benzoic acid and sorbic acid in fish oil-buffer system is ca. 100 times lower than that in fish oil-water system. The K ow values of benzoic acid and sorbic acid in fish oil and water system decrease with increasing system pH values. The partition coefficients of plant origin and fish origin oils are in the same order of magnitude even though their molecular structures are very different.