This work investigates the mixture formation process in a SI engine with air-assisted port fuel injection. The combination of this injection method with low flow velocities and wall temperatures, typical of engine starting and warm-up conditions, results in the build-up of a fuel film along the walls of the port. This in turn results in increased fuel consumption and high unburned hydrocarbon emissions.This investigation concluded that with the application of air-assisted injectors, the mixture formation process could be improved with a resulting reduction of the fuel-film build-up in the intake ports. Comparative measurements of the drop size distribution from conventional and air-assisted injectors showed that even very small amounts of air resulted in better fuel atomization. An experimental analysis of the effect of unsteady air flow on spray dispersion and fuel film creation characteristics of air supported injectors was undertaken for a straight test section. Measurements of the fuel-film in the intake port of a mass production cylinder head showed that a considerable decrease of the development of such fuel-films is possible through the use of air-assisted injectors.Through systematic analysis of the flow parameters in the test section it was possible to evaluate their effects on atomization and film generation. The comparison of this analysis with the measurements taken on the production cylinder head show the transferability of the model to the complex conditions in the intake port.