Abstract: Water sample containing the organoarsenic feed additive roxarsone (ROX) was photodegraded under simulated condition. An aliquot of the sample solution after photodegradation was taken and its total inorganic arsenic content was determined by HG-AFS. It was shown that: ① after UV radiation, ROX in the solution was degraded into poisonous inorganic arsenic compounds; ② rate of photodegradation was found to increase with increase of time of radiation; ③ optimum acidity of sample solution for degradation was found at pH 6; ④ co-existing NO3- ion was found to act as catalyst for degradation; ⑤ a smaller volume of ROX solution leads to higher rate of degradation.