Erythrosine, a popular food dye, undergoes fast O2-sensitive bleaching in water when subjected to visible light illumination. In dilute solution, erythrosine undergoes photobleaching via first-order kinetics, where the rate of bleaching depends critically on the rate of photon absorption and on the concentration of dissolved oxygen. Kinetic studies indicate that this inherent bleaching is augmented by self-catalysis at higher concentrations of erythrosine and on long exposure times. Under the conditions used, bleaching occurs by way of geminate attack of singlet molecular oxygen on the chromophore. Despite the complexity of the overall photobleaching process, the rate constants associated with both inherent and self-catalytic bleaching reactions follow Arrhenius-type behavior, allowing the activation parameters to be resolved. Bleaching remains reasonably efficient in the solid state, especially if the sample is damp, and provides a convenient means by which to construct a simple chemical actinometer.