A technique, based on the unique relationship that exists between the dielectric constant of a fully oxidized exhaust gas sample and the fuel:air ratio of the unburnt mixture, is presented. To measure the dielectric constant a special cell has been constructed through which the fully oxidized gases are passed. The capacitance of the cell is then directly proportional to the dielectric constant of the gases so that changes in mixture strength are detected as changes in cell capacitance. These changes in cell capacitance are measured electronically and are continuously recorded as an analogue voltage on a chart recorder. The cell design is such that the residence time of the gases is 1/10 s at a flow rate of 2 1/min-1.