Abstract: Parameters of a freeze drying process are monitored by measuring trace materials in the vacuum pump exhaust. The measurements are made using techniques such as acousto-optic spectrometry, multipass cavity-enhanced absorption spectrometry (CEAS) and cavity ring-down spectrometry (CRDS). The technique may be used to diagnose problems such as leakage in the freeze drying shelves, leakage in the condensation coils, leaks from atmosphere in the chambers and the presence of residual cleaning materials. The technique may also be used to monitor the presence of water vapor in the exhaust of a secondary drying process.