The aqueous photochemical fates of three pharmaceuticals and personal care products (PPCPs), cimetidine, ranitidine, and triclosan were examined, as was the microheterogeneous distribution of singlet oxygen in irradiated dissolved organic matter (DOM) solutions. Each of the three PPCPs examined were photolabile. Despite their similar structures, cimetidine and ranitidine displayed disparate photodegradation pathways. While cimetidine did not degrade by direct photolysis, ranitidine was quickly degraded by direct photolysis in sunlight (half-life = 35 min, 45 degrees latitude, noon summertime sunlight). In sunlit natural water, cimetidine was degraded in an indirect photolysis mechanism involving singlet oxygen. Experiments in natural waters also showed that the photodegradation rate of ranitidine was enhanced (ca. 10 percent) due to reaction with singlet oxygen. Studies with model compounds indicated that the heterocyclic rings of each drug are the active sites for singlet-oxygenation, while the nitroacetamidine moiety of ranitidine was the important chromophore in the direct photolysis mechanism. Triclosan, a high-use antimicrobial, was rapidly degraded by direct photolysis. Two toxic products, 2,8 dichlorodibenzo-p-dioxin and 2,4-dichlorophenol, were formed in low yields in the direct photolysis experiments. In photolysis experiments with added Suwanee River fulvic acid, some of the triclosan coupled to the DOM. Although high bimolecular reaction rate constants were measured for triclosan with singlet oxygen and hydroxyl radical (OH), indirect photolysis mechanisms are unlikely to compete with direct photodegradation because of the very low concentrations of singlet oxygen and OH in natural waters. The microheterogeneous distribution of singlet oxygen in irradiated aqueous DOM solutions was elucidated using hydrophobic trap-and-trigger chemiluminescent probe molecules in conjunction with the conventional hydrophilic probe furfuryl alcohol (FFA). The hydrophobic probes measured significantly higher singlet oxygen concentrations than did FFA, reflecting the relatively high local singlet oxygen concentration within the DOM. A three-region model was proposed to explain the microheterogeneous singlet oxygen distribution. In this model, singlet oxygen is produced solely in the DOM phase, and the aqueous phase is divided into two regions: a corona surrounding the DOM phase that receives diffusive singlet oxygen flux from the DOM region and the bulk water phase that has a negligible singlet oxygen concentration.