:The aim of this work is to provide a new tool for quantitative measurements of the fuel/air ratio in the combustion chamber of a spark ignition engine. This optical method is based on a new approach of Planar Laser Induced Fluorescence, taking advantage of the quenching phenomenon. A broadband KrF excimer laser (248 nm) induces the fluorescence of toluene to visualize the fuel/air distribution. At first, we made a spectroscopic study of the mixture iso-octane toluene in a cell. At last, in an SI engine equipped with optical access, instantaneous imaging of the fuel/air ratio was performed inside the cylinder during the compression stroke.