Abstract: A moisture indicator for indicating the state of moisture loading of an oxygen-releasing chemical in a respiratory protection device which contains the peroxide of an alkali metal as the substance is to be improved such that it will indicate an irreversible and clearly recognizable color change and will respond to the intrusion of even small amounts of moisture. To accomplish this task, the substance contains as the reactive substance an additive in the form of an alkali metal ozonide that engages in a color reaction with moisture present in the air.