Abstract: Detection of leakages of refrigerant from compressor-evaporator-condenser circuits is carried out with an electron multiplier equipped mass spectrometer of which the evacuated detector chamber is maintained at substantially constant elevated temperature in the range about 60 to about 130.degree. C. This allows high gain settings to be employed allowing detection of very small leakage rates by direct sampling from the ambient atmosphere while distinguishing from chemically analogous halogenated hydrocarbons used as blowing agents for foamed insulation and present in substantial concentrations in the ambient atmosphere.