A Duplex Gas Chromatograph was built to give complete analysis of products from gas phase photolysis and radiolysis reactions of simple organic compounds. Products which can be analyzed include permanent gases (H2, CH4, N2, etc.), less volatile components with boiling points up to 250°C, and inorganic or other components amenable to external analysis by spectrophotometry, titration, etc. The unit consists of two separate gas chromatographs and a common vacuum inlet system, which includes a removable aliquot tube for external sample analysis, two sample loops, and an automatic Toepler pump. The Toepler pump collects permanent gases in one sample loop for analysis in an isothermal chromatograph with a thermal conductivity detector, while less volatile components are trapped in a second sample loop for analysis in a temperature programmed chromatograph with a hydrogen flame ionization detector. To permit peak identification, the apparatus is equipped with a stream splitting valve to divert a portion of the sample into a mass spectrometer. To obtain