Samples of water were collected from the port of Gdynia and the Gulf of Gdansk (Southern Baltic Sea), and were artificially covered by oil film and stored at a constant temperature. One part of samples was stored in darkness and the second batch was placed under artificial light. Change in the composition of oil on the water surface was determined by means of gas chromatography. Results indicate a greater rate of degradation for the film which covered port water than water from the Gulf of Gdansk. It was revealed that light decreases the rate of biodegradation of an oil film.