A flow-injection analysis (FIA) method for the determination of chlorophenoxyacid herbicides, based on photochemically induced fluorescence (PIF) detection, is described. PTFE tubing was helically coiled around a low-pressure mercury lamp located between the injection valve and the detector. Two analytical procedures are used, either continuous or stopped-flow mode, according to the photochemical behaviour of the analyte. Ultraviolet (UV) photolysis in a 50 % (v/v) MeOH/pH 5 buffer solution provoked a more or less marked increase of the fluorescence signal depending of the herbicide structure. In the stopped-flow mode, UV irradiation times ranged between 90 sec and 12 min. Linear dynamic graphs were established over a concentration range of nearly two orders of magnitude. The FIA-PIF limits of detection ranged from 23 to 98 ng/ml, according to the compound. Relative standard deviations were between 0.7 and 2.7%. The proposed method has been applied to determine these herbicides in spiked river water samples, with mean recoveries ranging from 96 to 108%. It shows the usefulness of this technique for chlorophenoxyacid herbicides analysis.