The Gas Chromatograph Mass Spectrometer (GCMS) will be a versatile gas chemical analyser designed to identify and measure chemicals in Titan's atmosphere. It will be equipped with samplers that will be filled at high altitude for analysis. The mass spectrometer will build a model of the molecular masses of each gas, and a more powerful separation of molecular and isotopic species will be accomplished by the gas chromatograph. During descent, the GCMS will also analyse pyrolysis products passed to it from the Aerosol Collector Pyrolyser. Finally, the GCMS will measure the composition of Titan's surface in the event of a safe landing. This investigation will be made possible by heating the GCMS instrument just prior to impact in order to vaporise the surface material upon contact.