"The monitoring and control of the vacuum coating process is of utmost importance for consistent, high quality end-product. Monitoring of the gases present within the vacuum chamber is an essential component of this. Every process will at least have one pressure gauge and some will make use of a Residual Gas Analyser (RGA) to monitor the condition of the base vacuum before the start of the process. Very few systems will monitor the gases present during the process itself: this is due to high cost and complexity of differentially pumped RGAs that can operate at process pressures. For web coating systems this is particularly disadvantageous as the often delicate, chemically complex and variable nature of the web substrate can result in inconsistent deposited film quality without knowing the vacuum environment during coating. The gas sensing technique of Remote Optical Emission Spectroscopy (ROES), whereby a small plasma is generated inside a sensor in order to detect and monitor gases, can be used directly (and therefore cost effectively) to monitor gases at process pressures. Gencoa Limited have developed a novel sensor, called Optix, based on this technique. This paper presents results from using this sensor to monitor an AlOx reactively sputtered deposition process onto PET web. The results show how the sensor can be used to assess the state of the cathodes during the target cleaning step, characterise the reactive sputtering process during deposition and to monitor the effect that the deposition process has on the web substrate. Damage to the substrate via bombardment of activated oxygen during the reactive sputter step is also investigated by use of the sensor to monitor damage to the web via detection of by-products created during the bombardment. Control of damage to the substrate caused by activated oxygen is of critical importance for many sensitive substrates such as OLED devices or organic electronic devices. This paper also presents findings of the use of magnetically guided anodes to control the oxygen bombardment of the substrate – and thus to reduce substrate damage.