Under the activating influence of short-wave ultraviolet light, diphenylamine and nitrate ions in solution or on filter paper react to yield a yellow-colored product. Between pH 4 and 10 this consists mainly of 2-nitrodiphenylamine and 4-nitrodiphenylamine. Over this range of pH, the absorption maxima at 405 mμ did not shift; however, the intensity of absorption appeared to reach a minimum at or near neutrality and then increase rapidly as the solution was made more alkaline. The reaction rate was not affected by lowering the concentration of diphenylamine from 0.38% to 0.038%. A similar change in nitrate concentration decreased the rate 5.4 times. No evidence of nitroso compounds was found. Possible mechanisms are suggested.