Abstract: UV-irradiation has been known as an effective method for the decomposition of organic phosphorus compounds in water.In this paper,the conditions of photo-decomposition have been studied.We found that K₂S₂O₈ as an photo--oxidant is more effective than other oxidants (e.g.,H₂O₂ which is commonly used).The suitable pHof the medium is about 3.In the range of 217-650μg-P/1,all the compounds studied were quantitatively decomposed within 10 min.The relative standard deviation for the determination is less than_3.6%.The interferences during irradiation process were studied.Finally,the possibility of using this method to determine organic phosphorus in water samples is discussed.