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Single-wall carbon nanotubes (SWCNTs) have a variety of potential and demonstrated applications, and their production rates are increasing rapidly. This increase in production has motivated research on their transport and potential transformation and their toxicity in the environment. In this work, we examined the direct and indirect photoreactivity of SWCNTs under sunlight conditions. We found that the direct photoreactivity of pristine SWCNTs is generally low; however, indirect photoreaction involving center dot OH may be significant in natural aquatic environments. Environmental photochemical reactions generating center dot OH lead to distinct changes in SWCNT fluorescence efficiency in the near-infrared (NIR) region, Raman spectra, and light attenuation spectra in the UV, visible, and NIR regions, indicating that covalent functionalization of SWCNTs occurs. The reactivity of SWCNTs to center dot OH is dependent on the specific chiral structure of the SWCNTs and the surfactant associated with it. An operationally defined second-order rate constant (based on the decrease in NIR fluorescence signals) for all SWCNT chiral species reacting with center dot OH was estimated to be (2.91 +/- 1.30) x 10(10) M-1 s(-1). Our work suggests that photochemical reactions may be a significant transformation pathway of SWCNTs in aquatic systems.