The production of reactive oxygen species (ROS) by aqueous suspensions of fullerenes and nano-TiO2 (Degussa P25) was measured both in ultrapure water and in minimal Davis (MD) microbial growth medium. Fullerol (hydroxylated C60) produced singlet oxygen (1O2) in ultrapure water and both 1O2 and superoxide (O2−•) in MD medium, but no hydroxyl radicals (OH•) were detected in either case. PVP/C60 (C60 encapsulated with poly(N-vinylpyrrolidone)) was more efficient than fullerol in generating singlet oxygen and superoxide. However, two other aggregates of C60, namely THF/nC60 (prepared with tetrahydofuran as transitional solvent) and aqu/nC60 (prepared by vigorous stirring of C60 powder in water), were not photoactive. Nano-TiO2 (also present as aggregates) primarily produced hydroxyl radicals in pure water and superoxide in MD medium. Bacterial (Escherichia coli) toxicity tests suggest that, unlike nano-TiO2 which was exclusively phototoxic, the antibacterial activity of fullerene suspensions was linked to ROS production. Nano-TiO2 may be more efficient for water treatment involving UV or solar energy, to enhance contaminant oxidation and perhaps for disinfection. However, fullerol and PVP/C60 may be useful as water treatment agents targeting specific pollutants or microorganisms that are more sensitive to either superoxide or singlet oxygen.