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Natural organic matter (NOM) present in surface water sources undergoes significant structural and chemical changes during the application of vacuum ultraviolet (VUV)-based advanced oxidation process (AOP), which leads to the formation of undesired by-products such as aldehydes. Partial NOM mineralization can occur depending on the fluence received. In this paper, the VUV-induced degradation of NOM and the subsequent formation of by-products were experimentally studied. Two natural waters (Trepanier Creek and Capilano Reservoir, in British Columbia, Canada) and two synthetic waters (Suwannee River, Southern Georgia and Northern Florida; Nordic Reservoir, Vallsjøen, Skarnes, Norway) were used. While the process started with partial oxidation of NOM, extensive irradiation (i.e., high fluence) led to complete mineralization of NOM. The rates of mineralization were greatly dependent on the nature of NOM and on the presence of inorganic compounds in the water matrix, such as carbonates and bicarbonates. A faster mineralization rate was obtained with waters containing NOM with a lower average molecular weight. Size exclusion chromatography analysis showed that high-molecular-weight (HMW) molecules were readily degraded, even from the beginning of the treatment. The fraction of low-molecular-weight (LMW) compounds also reduced, but at a slower rate, largely because LMW compounds were both formed through partial degradation of HMW compounds and degraded by hydroxyl radicals. The concentration of aldehydes and trihalomethane formation potentials increased up to 120-700 ppb, respectively, after receiving a fluences in the range of ), but decreased with relatively higher fluencies (). [ABSTRACT FROM AUTHOR]