A photoreduction - fluorescence technique has been developed for the quantitative analysis of several dinitroaniline derivative herbicides. The photoreduction of anthraquinone - 2.6 - disulfonate by dinitroanilines generates highly fluorescent dihydroxyanthracene-2,6disulfonate, in a 20 : 80 vol/vol acetonitrile-water mixture at room temperature. Optimal irradiation times for obtaining a convenient, sensitized fluorescence signal are relatively short, ranging between 2 and 4 min.