The photodecomposition of 2,4-dichlorophenoxyacetic acid (2,4-D) in aqueous solutions has been studied by using ultraviolet radiation in an annular batch reactor. The dark reactions were negligible. For short irradiation times, a first order reaction was found with respect to the absorbed radiation and a simple mechanism of this reaction at initial times has been proposed. pH influences the reaction rate only for long times of irradiation, possibly due to the formation of humic acids in the reaction medium.