The parameters affecting the onset of flashing in microjets were studied. A custom atomizer assembly was designed to produce jets of propellant mixtures at different temperatures. Propellants chosen were HFA134a and HFA227ea, which are used in most pressurized metered dose inhalers (pMDIs). Ethanol was added to the propellants to investigate its effect on flashing. Orifices used ranged from 5 to 35 μm in diameter. A critical jet diameter above which flashing was the dominant mechanism of atomization was found at each temperature and formulation. A correlation was developed to relate the critical jet diameter to the thermophysical properties of the formulation.