During frying, the degradation of oil produces harmful compounds. Improper monitoring of oil discard times in restaurants either risks the public health or causes financial losses to industries. Measuring the oil quality is a complex problem and a satisfactory sensor has not been developed yet. Kinetics of oil degradation and various factors influencing it during Frying were studied. Concentrations of major degradation products were measured. Changes in various physical properties and analytical indices of frying oil were determined. Dielectric constant and optical properties correlated well with oil degradation. A new method of oil quality measurement was developed. This led to the invention of a Frying Oil Quality Sensor. The sensor can solve complex measurement and calibration problems incorporating many automated features.