The oxidative breakdown of lubricating oils is accompanied by by monitoring volatile oxidation products with an electronic the liberation of vapor-phase oxidation products. A simple apparatus which uses a semiconducting tin oxide gas sensor has been employed to monitor the evolution of these products and determine the length of the induction period.The method provided reliable end-point detection with ester-base lubricants of moderate stability, simplifying the oxidation testing procedure and providing useful data for studies of lubricant stability.A sensor calibration technique was developed, and the sensors were found to have adequate sensitivity, response speed, and reliability.