Electrochemical zircon dioxide oxygen sensors were applied to measure oxygen in diamond hot-filament CVD (HFCVD) coating atmospheres. These low-pressure hydrogen environments are highly reductive and methane containing. Under industrial coating conditions there is the possibility that oxygen from ambient air enters the reactor by leakages. This leads to a tremendous influence on coating conditions by lowering the active carbon content in the gas phase and by decarburizing the gas activating filaments in HFCVD. In order to ensure process stability, a monitoring of an oxygen-related value is desired. This work displays the possibility to measure oxygen in HFCVD with potentiometric and amperometric ZrO 2 sensors. The dependency of the sensor signals on process parameters is investigated and the possibility of monitoring an oxygen-related value during a coating process is shown. The sensor signals allow a comparison of plants and processes regarding the seal tightness condition. The employed zircon dioxide sensors offer a cost effective and feasible method to monitor air leakage and ensure process reproducibility.