Abstract: The invention relates to a method for determining and/or monitoring the air tightness of an enclosed room (2) which is equipped with an oxygen reducing system (1) and in the atmosphere of which at least one oxygen content that can preferably be determined in advance and is reduced in comparison to the normal surrounding air can be set and maintained in order to prevent and/or extinguish fires by introducing an oxygen-displacing gas. The oxygen reducing system (1) has a compressor system (4; 4.1, 4.2) for compressing an initial gas mixture and a gas separation system (3; 3.1, 3.2) downstream of the compressor system (4; 4.1, 4.2) for separating at least one part of the oxygen contained in the initial gas mixture and for providing a nitrogen-enriched gas which is supplied to the enclosed room (2). The differential pressure set in the room (2) is ascertained and compared to a corresponding reference value, whereby information regarding the air tightness of the room (2) is provided.