Abstract: A device for monitoring the stack exit air in a reactor installation consists essentially of a pressure relief line (2) which connects the containment vessel (1) to the stack (5) and in which a filter unit (3) is located. Downstream of the filter, a sampling point (6) is provided in the pressure relief line, from where gas mixture is branched off via a sampling line (7). This gas is passed through a measurement section (14) and then returned into the pressure relief line (2). The concentration of the gas is reduced in a dilution unit (8) upstream of the measurement section (14). Cooling of the gas sample in the sampling line (7) before dilution is prevented by means of a heater (24).