The effect of the photosensitizer riboflavin (0, 10, 25, 50, 100 microM) and a river humic acid (0, 20, 40, 80 ppm) on the photolysis of 6-aminochrysene (6AC) in 10 mM PBS solution (pH 7.0) was studied. The presence of riboflavin significantly enhanced photolysis rate of 6AC. The photo-transformation half-life of 6AC was 1 or 36 min, respectively, in the presence or absence of riboflavin. The humic acid inhibited the photo-transformation rate of 6AC. The photo-transformation half-lives of 6AC were 37, 56, 92 min at 20, 40, 80 ppm humic acid, respectively. By using LC-MS, the main 6AC photoproduct identified was 5,6-chrysene-quinone along with some minor products. Both 6AC and 5,6-chrysene-quinone exhibited photoinduced cytotoxicity. A photochemical transformation pathway for 6AC was derived.