In this study, photochemical degradation of two emerging pharmaceutical chemicals, mefenamic acid (MF) and triclosan (TCS), was investigated to clarify the role of treated wastewater effluent matrices on their environmental photolysis. Target compounds were individually exposed to simulated sunlight in different media: ultrapure buffered water and synthetic field water with treated municipal wastewater effluent. The results in ultrapure buffered water showed that the direct photolysis processes in aquatic environments are not relevant to the elimination of MF. However, in samples containing treated wastewater effluent, photochemical degradation of MF was clearly enhanced. Our results indicate that MF undergoes indirect photolysis by reactive intermediates produced in an effluent matrix. Further quenching experiments suggested that photochemically produced hydroxyl radicals and excited triplet state dissolved organic matter drive the degradation of MF. In contrast to MF, TCS photochemical degradation proceeds through rapid direct photolysis. TCS was quickly degraded in ultrapure buffered water but it is considerably hampered in samples containing wastewater effluent. The declined degradation of TCS in the synthetic field water was discussed in terms of underlying optical filter effects by coexisting chromophoric substances. Results emphasize the importance of taking local water chemistry into consideration when predicting natural attenuation of pharmaceutical chemicals in receiving areas.