This work examines flash atomization of water and water/acetone by varying the relative concentrations of propellant gas and liquid, injection temperature, and pressure. Breakup pattern and spray quality were characterized by taking images at the nozzle exit. Mean droplet diameters were measured as a function of operating conditions. It is shown that acetone as a propellant liquid can significantly enhance the atomization of water. Further enhancement can be achieved by adding a propellant gas (nitrogen). For a given mean drop size, the presence of the propellant liquid can markedly reduce the propellant gas-to-liquid ratio. This result has a direct bearing on the atomization of heavy hydrocarbon liquids with injectors subject to propellant gas and/or pressure constraints.