Abstract Photobleaching of the aqueous alcoholic erythrosin solutions in air and in vacuo was investigated. In general, erythrosin showed behavior similar to eosin. An excited dye molecule which participates in the primary processes is the long-lived species, having a lifetime of the order of 10−7sec. It was concluded that the primary process in aerobic aqueous erythrosin solution proceeds via the process, D+O2→D······O2, as in the case of eosin. It was found that excited erythrosin is somewhat more reactive than excited eosin towards dissolved oxygen, while it is less reactive towards alcohols in vacuo. Some quantitative values on the reactivities were given.