VAPOUR phase chromatography has been used with great success in these laboratories for the analysis of the products of reactions in the gas phase. By using a thermal conductivity detector it has been possible to estimate amounts of product of the order of 10-8 mole; but this was the lowest limit which could be reached. In certain cases, however, it became necessary to detect 10-12 mole of a substance. The detector described below has been developed to meet this requirement. It is of wide applicability and is easily constructed.