Nonenveloped viruses are shown to be inactivated by singlet oxygen (1O2) produced in UVA photosensitized aqueous suspensions of a polyhydroxylated fullerene (C60(OH)22‚àí24; fullerol, 40 ŒºM). Experiments were performed with MS2, a ssRNA bacteriophage, as well as two dsDNA phages: PRD1, which has an internal lipid membrane, and T7, which entirely lacks lipids. MS2 was highly susceptible to inactivation, having a rate constant of 0.034 min‚àí1 with UVA alone, which increased to 0.102 min‚àí1 with photoactivated fullerol. PRD1 and T7 were not susceptible to UVA alone but were photoinactivated by fullerol with rate constants of 0.026 and 0.035 min‚àí1, respectively. The role of 1O2 was demonstrated by three independent observations: (i) viruses that were insensitive to UVA alone were photoinactivated by rose bengal in the absence of fullerol, (ii) Œ≤-carotene reduced (but did not eliminate) photoinactivation rates, and (iii) singlet oxygen sensor green fluorescence spectroscopy directly detected 1O2 in UVA illuminated fullerol suspensions. Qualitative evidence is also presented that fullerol aggregates were closely associated with viruses allowing efficient transfer of 1O2 to their capsids. Fourier transform infrared spectroscopy revealed significant oxidative modifications to capsid proteins but comparatively minor changes to the DNA and (phospho)lipids. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) suggested 1O2 induced cross-linking of proteins. Hence, phage inactivation by photoactivated fullerol nanoparticles appears to be caused by cross-linking of capsid protein secondary structures by exogenous 1O2 and consequent impairment of their ability to bind to surface receptors of their bacterial hosts (loss of infectivity) rather than by direct reactions with fullerol.