A method to determine diphenylamine (DPA) in aqueous solution by measuring chemiluminescence intensities using a stopped-flow system has been studied. The lucigenin-hydrogen peroxide chemiluminescence reaction was chosen for the determination of diphenylamine. The chemiluminescence intensity from the lucigenin system increased with the addition of diphenylamine. Under the optimum experimental conditions, the calibration curve was linear over the range 1.0 X 10^-5 M for diphenylamine and the correlation coefficient was 0.99864. The detection limit (S/N=3) was 3.0 X 10^-10 M.