A sensor system (36) determines the Stoichiometric Air to Fuel Ratio (SAFR) of fuel mixtures. The system includes a first electrode (12) and a second electrode (14), with the first electrode surrounding the second electrode so that a fuel mixture can flow between the first electrode and the second electrode. The electrodes are constructed and arranged to provide data for determining a conductivity and permittivity of the fuel mixture. A temperature sensor (18) is constructed and arranged to measure a temperature of the fuel mixture. A processor (19) is constructed and arranged to determine the SAFR of the fuel mixture based on the measured temperature and permittivity of the fuel mixture.