Spray formation by bi-component liquid flashing through a special-design injector has been studied. Special attention has been drawn to the processes inside the expansion chamber. The relevant processes, which include the pressure drop at the inlet orifice, nuclei formation, bubble growth inside the expansion chamber, and the pressure drop at the discharge orifice, have been analyzed by using a one-dimensional model approach. While the one-dimensional assumption is problematic, it enables simple analysis and yet provides realistic quantitative results. It is postulated that in a well-designed expansion chamber, a prespecified void fraction has to be attained at the end of the expansion chamber. The latter is designed to yield this void fraction, subject to the thermodynamic conditions of the entering mixture and orifices geometries. The optimal volume of the expansion chamber is found to be [GRAPHICS] It follows that the optimal volume of the expansion chamber depends strongly on the superheat degree, flow rate, and the cross-section area ratio between the inlet and discharge orifices.