A method for determining and/or monitoring the airtightness of an enclosed room equipped with an oxygen-reducing system in which at least one level of oxygen which is reduced in comparison to the normal ambient air can preferably be preset and maintained in the spatial atmosphere of the room by introducing an oxygen-displacing gas, wherein the oxygen-reducing system comprises a compressor system (4; 4.1, 4.2) for compressing an initial gas mixture containing at least nitrogen and oxygen and a gas separation system (3; 3.1, 3.2) downstream of the compressor system (4; 4.1, 4.2) for separating off at least a portion of the oxygen contained in the initial gas mixture and for providing a nitrogen-enriched gas at the outlet of the gas separation system (3; 3.1, 3.2) which can be fed into the enclosed room to set and/or maintain a reduced oxygen content, wherein the method comprises the following method steps: i) providing fresh air from outside of the enclosed room as the initial gas mixture; ii) compressing the provided fresh air in the compressor system (4; 4.1, 4.2) of the oxygen-reducing system; iii) feeding the compressed fresh air to the gas separation system (3; 3.1, 3.2) of the oxygen-reducing system and separating off at least a portion of the oxygen contained in the fresh air in the gas separation system (3; 3.1, 3.2) such that a nitrogen-enriched gas is provided at the outlet of said gas separation system (3; 3.1, 3.2); iv) introducing the nitrogen-enriched gas provided at the outlet of the gas separation system (3; 3.1, 3.2) into the enclosed room; v) determining the pressure prevailing in the enclosed room or set in the enclosed room relative to the ambient air pressure; characterized by the method steps of: vi) comparing at least one differential pressure value determined in method step v) to a corresponding reference value; and vii) evaluating the comparative result obtained in method step vi), wherein depending on the amount of deviation between the differential pressure value determined and the corresponding reference value, a conclusion is made as to the airtightness of the enclosed room and/or monitoring of the airtightness of the enclosed room occurs. i) providing fresh air from outside of the enclosed room as the initial gas mixture; ii) compressing the provided fresh air in the compressor system (4; 4.1, 4.2) of the oxygen-reducing system; iii) feeding the compressed fresh air to the gas separation system (3; 3.1, 3.2) of the oxygen-reducing system and separating off at least a portion of the oxygen contained in the fresh air in the gas separation system (3; 3.1, 3.2) such that a nitrogen-enriched gas is provided at the outlet of said gas separation system (3; 3.1, 3.2); iv) introducing the nitrogen-enriched gas provided at the outlet of the gas separation system (3; 3.1, 3.2) into the enclosed room; v) determining the pressure prevailing in the enclosed room or set in the enclosed room relative to the ambient air pressure; vi) comparing at least one differential pressure value determined in method step v) to a corresponding reference value; and vii) evaluating the comparative result obtained in method step vi), wherein depending on the amount of deviation between the differential pressure value determined and the corresponding reference value, a conclusion is made as to the airtightness of the enclosed room and/or monitoring of the airtightness of the enclosed room occurs.