Spray formation by fuel-dissolved gas atomization is studied. The geometry of the injector, which consists of two orifices separated by an expansion chamber, was optimized to produce spray with minimal Sauter mean diameter (SMD). Special attention is drawn to the flow pattern inside the expansion chamber and on its significant impact on the resultant spray quality. Three possible characteristic regimes were identified: all liquid flow, stratified two-phase flow, and well-mixed bubbly flow. The latter regime results in the lowest SMD prays. The necessary operation conditions and injector geometry in order to reach the well-mixed bubbly flow regime were mapped. A possible explanation for the optimal value of about 0.7 for the orifice diameter ratio has been proposed When optimizing an injector of this type, it is anticipated that the pressure of the expansion chamber plays an important role. The pressure is primarily determined by the orifice diameter ratio. A lower pressure enhances both the nucleation rate at the inlet orifice and the bubble growth rate inside the chamber. Conversely, a higher pressure enhances the discharge velocity and the bubble growth rate downstream of the discharge orfice; both result in a better secondary droplet breakup. In an attempt to find an empirical criterion for the optimal orifice diameter ratio, we examined various combinations of the relevant parameters. The product of the void fraction and the pressure drop across the discharge orfice has been found fairly useful over the entire range of the present experimental conditions.