Instability mechanisms discussed to date are unable to account for the behavior of long‐pulse self‐sustained discharge‐pumped XeCl devices. We develop theoretical arguments supporting the concept that halogen donor depletion during the discharge pulse is the principal cause of discharge collapse and the termination of lasing. The theoretical prediction that the duration of the glow phase should vary as the inverse square of the initial halogen donor concentration is verified by experimental results furnished by a study of a resistively stabilized long‐pulse xenon chloride laser.