Dissolved Organic Matter (DOM) is a complex mixture of thousands of organic molecules ubiquitously present in surface waters. Its influence on the photodegradation of organic contaminants is a complicated topic. For decades DOM had been mostly assumed to have a positive effect (i.e., promoting the degradation of contaminants), but more recently it was discovered that for some compounds, DOM can also have negative effects (i.e., inhibiting the photodegradation of contaminants) in surface waters. These dual properties of DOM are the subject of the investigations of this thesis, in which DOM is considered for both its photosensitizing and inhibitory properties. The goal of this thesis is to achieve a better understanding of the photoreactivity of DOM towards contaminants in surface waters. To investigate the dual effect of DOM, an initial screening study was done to look for an optimal probe compound, subject to both photosensitizing and inhibitory effects. Nine candidate pharmaceuticals or model compounds, almost exclusively aromatic amines, were selected and their photodegradation evaluated under simulated sunlight in various water matrix. This study lead to the selec-tion of N,N-dimethyl-4-cyanoaniline (DMABN) as a probe compound. The indirect photodegradation of DMABN was characterized and shown to proceed mostly through excited triplet states of DOM. The indirect photodegradation kinetics of DMABN was also studied in various waters and mixtures of waters, and model equations describing the effect of the photosensitizing and inhibitory properties of DOM on photodegradation rates constants were developed. An indirect phototransformation pathway for DMABN was proposed, in which following initial oxidation of DMABN by triplet DOM, deprotonation of the formed radical cation of DMABN (DMABN+.) occurred. Subsequently, demethylation of DMABN took place, forming N-methyl-4-cyanoaniline and formaldehyde. The absence of deuterium kinetic isotope effect on the inhibitory effect of phenol, used as a surrogate for DOM, on the indirect photo-transformation of DMABN indicated that the inhibition proceeds by electron transfer. To study the inhibitory effect of DOM, laser flash photolysis (LFP) investigations to detect short-lived reaction intermediates and measure their reaction rate constants were performed. The rate constant for DMABN-induced quenching of a series of triplet-aromatic ketones, used as proxies for excited triplet DOM were found to follow well-known relationships in the frame of Marcus theory of electron transfer. DMABN+. was observed on a microsecond time scale as the primary species resulting from the reaction between the excited triplet photosensitizers and DMABN. The decay kinetics of this radical cation was studied in the presence and absence of phenols or DOM. DMABN+. was found, as expected, to be quenched by both phenols and DOM, confirming the postu-lated model for the inhibitory effect. Electron-rich phenols reacted faster than phenol with DMABN+., while the rate constants for quenching by DOM were proportional to the electron donating capacity of the DOM. Finally, a similar study as for DMABN was performed with sulfadiazine, a well-known antibiotic, as target compound. Results were qualitatively similar to those for DMABN but their interpretation was more complex probably due to deprotonation of the intermediate radical product.