Abstract: Sample collecting system for gaseous or vaporous condensable radioactive substances contained in the ambient air, especially for collecting traces of tritium via the T.sub.2 O vapor content in the exhaust air of a nuclear plant. A gas stream of exhaust air is filtered and the filtered gas continues its flow through a suction branch line containing a measuring gas pump which propels the gas stream into and through a precipitate line. The precipitate line, enclosed in a measuring-gas cooler which is a cooling chamber, extends downward and then upward and then out of the cooling chamber, through a measuring-gas choke, through a flow transducer and the gas stream then returned to the exhaust gas flue or ambient air. The pump feeding the precipitate line has its pressure maintained constant by a bypass line and a pressure difference control valve from the output side of the flow transducer to the discharge side of the pump.