Metal oxide nanoparticles have been used in several systems as photocatalysts recently. Light initiated degradation of the organic compounds is one of the most important application of these photocatalytic systems. Photocatalytic degradation of organic compounds in waters and wastewaters has been widely studied for many decades. TiO2 is the most widely used photocatalyst due to its decent photocatalytic properties, ease to production and low cost. Major disadvantage of TiO2 photocatalyst is its bandgap energy (Ebg=3.2 eV, ??390 nm) for solar light activation since only 5% of solar light is UV light that can produce equal energy to TiO2 band gap energy. Therefore, effective solar light utilization can be achieved with modifications through doping and/or coupling with other semiconductors such as ZnO. As an organic compound, natural organic matter degradation by photocatalysis has awoken interest for treatment systems and studied extremely in the last decade. Recently, Bekbolet and co-workers has been working on photocatalytic degradation of natural organic matter under solar irradiation, aiming to improve photocatalytic degradation yield with second generation photocatalysts. Beside the treatment efficiency, interactions between organic matter and metal oxides during photochemical reactions has also importance to understand possible further environmental applications. Formation of aggregates, a particle comprising of strongly bound or fused particles, and/or agglomerates, a collection of weakly bound particles, changes the system dynamics, so investigation of these interactions is essential. Furthermore, introduction of these particles to natural waters brings the question of “behavior nanoparticles in aqueous medium” that deserves attention to be elucidated in terms of diverse reactivities leading to deterioration of water quality. Elucidation of the interactions between metal oxide nanoparticles and natural organic matter will be done by using TiO2 P25 as model nanoparticle and Suwannee River natural organic matter as organic matter. Characteristics of the binary system prior to, during and under post treatment conditions will be examined by UV-vis and fluorescence spectroscopy and physico-chemical properties will be assessed by particle size and zeta potential analysis and sedimentation experiments. The major outcome of the project would be to bring novel information to the understanding of the complex colloidal system operating under light leading to degradation of organic matrix via photolytic and photocatalytic oxidation pathways.