Abstract Investigation of the photobleaching of uranin solutions has led to the same conclusions as for eosin and erythrosin, concerning the primary processes both in aqueous and in alcoholic media. Strongly alkaline solutions of uranin in aqueous alcohol show an irreversible photobleaching in vacuo. On the reactivities of the excited uranin, and on the comparison of them with those of two other xanthene dyes previously investigated, some discussions have been given.