To assess the effectiveness of the advanced oxidation process (AOP), treated wastewater samples after reverse osmosis (ROP) and ultraviolet light/hydrogen peroxide (UVP) were analyzed by a non-targeted qualitative approach for the presence of organic compounds. Five sampling events were analyzed that occurred at Orange County Water District (OCWD)'s Advanced Water Purification Facility (AWPF) in Fountain Valley, CA. Tap water samples were also collected from a kitchen faucet at OCWD for two of the sampling events. Triplicates for each sample type were collected. The water samples were prepared by solid-phase extraction (SPE) and analyzed by comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry (GC_GC/TOF-MS). While targeted analysis approaches are used to detect known chemicals, non-targeted approaches provide a broad overview of all those that are detected, and consider unknown chemicals. In this study, true detection of a chemical was determined by meeting two criteria: it must be found in all triplicates and must not be found in any of the field blanks. Tentatively identified chemicals were determined by meeting the first two criteria and also meeting a mass spectral similarity score of 700 or above. On average, 71% of the number of chemicals detected in ROP were reduced after UVP among all sampling events. Fourteen compounds were detected and tentatively identified in UVP and also found in ROP. Majority of the 14 compounds were reduced after UVP by an average of 52.2% of their peak areas. The compounds that were most frequently found in UVP were two halogenated acetonitriles, dichloroacetonitrile and bromochloroacetonitrile, in which other studies have commonly found after AOP. The number of chemicals detected in tap water was significantly higher than the number in UVP. In the tap water sampling events, 4-chlorobenzotrifluoride (CBTF) was detected in high abundance and was further confirmed with an authentic standard. Possible sources of the trace chemicals were discussed. None of the compounds identified in this study is under regulation. This study concludes that AOP was effective to remove trace chemicals in ROP. Further study requires confirmation of the tentatively identified compounds and quantitation of the compounds for risk assessment and source investigation. The CD-ROM, an appendix to the thesis, is available for viewing at the Media Center in the SDSU Library.