Here we report an unexpected brightening of the fluorescence QY for individual SWNTs by up to an order of magnitude in aqueous solution, resulting in fluorophores having brightness equivalent to that of semiconductor quantum dots (QDs). The enhancement occurred upon addition of reducing agents to the SWNT suspensions, including dithiothreitol (DTT), Trolox, and β-mercaptoethanol (BME). The effect is reversible, suggesting that the interaction between the reducing molecules and the nanotubes is noncovalent. This surprising discovery is believed to arise from the passivation of defective SWNTs by reductant molecules that donate electrons to trap sites along the nanotube sidewall, which is thought to be predominantly doped by holes.(10, 15-17)