The different subsystems of the JET Active Gas Handling System (AGHS) are equipped with sufficient instrumentation for process monitoring and control during normal operations. In addition, especially for the commissioning phase and in case of process malfunctions a central facility was designed which is connected to all subsystems of AGHS and uses mass spectroscopy, gas chromatography and ionisation chambers for characterisation of the various gas mixtures. Furthermore, a simple oxygen monitor is presented which works reliably in gas mixtures containing mainly hydrogen at low pressures.