A method is described for quantitative determination of specific organic substances in the air in the presence of other, highly-concentrated air impurities. The required selectivity is achieved by the application of a coupled gas chro-matograph-mass spectrometer system. By concentration of the organic material from a one liter sample volume of air, reproducible detection limits of 1 ppb (10−9) have been obtained. Application of the method for the specific detection of bis(chloro-methyl) ether and dimethylsulfate is discussed. The proposed threshold limit values of these substances are 1 and 10 ppb, respectively. Bis(chloromethyl) ether is formed as a byproduct in chloromethylation reactions; dimethylsulfate has been manufactured for many years in large quantities and is used in methylation reactions. A reliable air monitoring system in both these important industrial processes is of great importance.