Photochemical processing of dissolved organic matter (DOM) across aquatic ecosystems is closely tied to the production of greenhouse gases, molecular transformation of DOM, supply of particulate organic matter to sediments, transport of modified terrestrial DOM to the ocean, and the complexation between DOM and trace metals. Previous studies have shown that terrestrial DOM can be removed from aquatic phase to form particles when exposed to simulated sunlight, i.e., photo-flocculation process. While the initial and final DOM, and the photo-produced particles were characterized in details from previous studies, significant information regarding the evolution of molecular size and composition signatures of DOM during photo-flocculation was not well examined until now. Our results reveal dynamic change of colloidal DOM during photochemical processing, and have great implication of the fate of organic ligands that can complex with metals.