Sunlight irradiation of 3,4-dichloroaniline (DCA) in natural water samples resulted in both direct and sensitized photoreactions. A procedure is described for separating the effects of competitive light absorption of the water from sensitized processes. Althought the overally photolysis rates of DCA were slower in the natural waters than in distilled water (depth 6.2 cm), the rates in natural waters were faster than expected, on the basis of sunlight received by the DCA. Kinetic results were qualitatively the same when a commercial humic acid solution with similar light absorption characteristics was used. TCAB (3,4,3',4'-tetrachloroacobenzene) was produced in two rice-field waters containing .0000061 to .00029 M DCA. The concentration of TCAB produced was greater in the more concentrated DCA solutions, although the yields were in no case greater than 1% of the reacted DCA. The chemical yield of TCAB was considerably lower in the commercial humic acid solutions than in the natural water samples. TCAB was not observed in distilled water. (Copyright (c) 1980 by the American Chemical Society.)