Irreversible photobleaching imparts the unavoidable limitation on single-molecule spectroscopy: when the fluorescent probe molecule becomes nonfluorescent, the experiment is over. A detailed study of the photobleaching rate for rhodamine 6G under vacuum reveals that low excitation rates yield long photochemical survival times and unprecedented numbers of emitted photons. A four-level system is used to model the photobleaching rate, reproducing the experimental bleaching rate and photon yield from single molecule and ensemble experiments. A higher triplet excited state T-n is found to be the predominant reactive state for photobleaching. This model produces a calculated quantum yield for photobleaching on the order of 7 x 10(-7) bleaching events per excitation to T-n. (C) 2002 Elsevier Science B.V. All rights reserved.