Engineered nanomaterials may undergo transformation upon interactions with various environmental factors. In this study, photochemical transformation of aqueous nC60 was investigated under UVA irradiation. nC60 underwent photochemical transformation in the presence of dissolved O2, resulting in surface oxygenation and hydroxylation as demonstrated by XPS and ATR-FTIR analyses. The reaction followed a pseudo-first order rate law with the apparent reaction rate constant of 2.2 x 10(-2) h(-1). However, the core of the nanoparticles remained intact over 21 days of irradiation. Although no mineralization or dissolution of nC60 was observed, experiments using fullerol as a reference fullerene derivative suggested likely dissolution and partial mineralization of nC60 under long-term UVA exposure. Aquatic humic acid reduced nC60 transformation kinetics presumably due to scavenging of reactive oxygen species. Results from this study imply that photochemical transformation is an important factor controlling nC60 physical and chemical properties as well as its fate and transport in the natural aqueous environment. In addition, changes in nC60 surface chemistry drastically reduced C60 extraction efficiency by toluene, suggesting that the existing analytical method for C60 may not be applicable to environmental samples.