The toxic chemicals, 2,4-D (a herbicide) and propoxur (an insecticide), were used as the model compounds in these experiments. Total organic carbon analyzer was used to assess the efficiency of photocatalytic mineralization. Microtox bioassay was employed in evaluating the toxicity of solutions treated by photocatalysis. Ultraviolet absorption spectra were also used for showing the different characteristics of the compounds undergoing photocatalytic oxidation. Results show that propoxur is less degradable than 2,4-D, and the photomineralization of these pesticides follows a behavior of first-order reaction. Products of 2,4-D and propoxur are more toxic than the parent compound after partial photodegradation. This shows that complete mineralization is necessary for total detoxification of these pesticides. In other words, toxicity is an important criteria in assessing the pretreatment process.