A laser induced fluorescence (LIF) technique to visualize the gasoline distribution in port injection spark ignition engines is presented. A dopant is selected according to a list of criteria required to make it a good tracer of the gasoline. This dopant, biacetyl, is shown to provide quantitative measurement in relative value. The LIF technique is applied for two-dimensional laser sheet imaging in a model engine with optical access. Extensive data processing is developped and used for a quantitative characterization of the mixing level. The effect of compression, injection timing, engine speed, and type of flow field on the mixture homogeneity is investigated.