<div class="htmlview paragraph">The laser-induced exciplex fluorescence (LIEF) technique, currently used to observe mixture formation in a diesel engine, has been applied to a spark ignition (SI) engine and a new equivalence ratio calibration technique has been developed in order that two-dimensional measurements of the equivalence ratio may be made in an operating engine.</div> <div class="htmlview paragraph">Spectrally separated fluorescent images of liquid and vapor phase fuel distributions were obtained by adding new exciplex-forming dopants to the gasoline fuel. Dual light sheets from an excimer laser were introduced into one of the cylinders of a 4-valve lean-burn engine, and 2-D images of the mixture formation were recorded at pre-set crank angles during the induction and compression strokes by an image-intensified camera equipped with the appropriate filter.</div> <div class="htmlview paragraph">The relationship between fluorescence intensity and equivalence ratio was investigated for purposes of calibration so that vapor phase spatial concentration maps of the equivalence ratio could be obtained for a quantitative analysis of mixture formation.</div>