Mixture strength, or equivalence ratio, is the primary factor affecting the fuel consumption and exhaust emissions of a gasoline engine. A new instrument has recently been developed to monitor equivalence ratio. The response time of the instrument is 1/2 second, so that the effects of engine transient conditions, such as fast throttle opening, can be studied with greater accuracy than is possible with current exhaust emission analysers.This paper will cover the concept, design and development of the prototype instrument, including the calibration tests on gas mixtures and an engine. In addition, a more sophisticated meter has been designed for production. The equivalence ratio characteristics of a number of current and advanced engines have been monitored with this instrument and the results are presented.