The transient behaviour of the fuel spray from an air assisted fuel injector has been investigated both numerically and experimentally in a Constant Volume Chamber (CVC) and an optical engine. This two phase injector is difficult to analyse numerically and experimentally because of the strong coupling between the gas and liquid phases. The gas driven atomization of liquid fuel involves liquid film formation, separation and break up and also liquid droplet coalescence, break up, splashing, bouncing, evaporation and collision. Furthermore, the liquid phase is the dominant phase in many regions within the injector.Experimental results are obtained by using Mie scattering, Laser Induced Fluorescence (LIF) and Laser Sheet Drop sizing (LSD) techniques. Computational results are obtained by using a mixed Lagrangian/Eulerian approach in a commercial Computational Fluid Dynamic (CFD) code. Injector rig results show a good atomization of the spray with low spray width and penetration and Sauter Mean Diameters (SMD) of droplets of order 10 µm. Engine results show a spray with a relatively low penetration producing liquid fuel and vapour fuel concentrations close to the central location of the injector and spark plug for stratified, lean mode of operation. Imaging of this injector operating inside an engine confirms the overall design approach for stratified operation.