In this work the effect of flash boiling on liquid breakup and atomization is first reviewed and then described in detail for the sprays produced by two high-pressure multistream gasoline direct injectors. They were operated at 120 and 200 bar pressure with a fuel injection duration of 0.8 ms into subatmospheric pressures between 1 and 0.1 bar and four temperatures between 20 and 100 degrees C. The sprays were injected into an optically accessed chamber whose temperature and pressure were independently controlled to recreate the conditions found in an engine likely to cause flash boiling. Shadowgraph imaging with a single-shot charge-coupled device camera was applied to obtain images of the spray morphology under a matrix of the above conditions. The interaction between spray streams is particularly complex under flash boiling conditions being a function of the nature of the fuel and the geometry of the injector. In this investigation, three different fuels, regular RON-95 gasoline and single components fuels, n-heptane and iso-octane, were used with two different injectors, one with a 60 degree and the other with a 90 degree nominal external cone angle. In total, five variables were investigated: fuel pressure, ambient pressure, ambient temperature, fuel composition, and injector geometry. Once processed, the recorded images allowed measurement of spray tip penetration and cone angle. Qualitative data on the change in shape of the spray were also available. The results showed that flash boiling has potential to reduce droplet diameters and improve fuel vaporization; however, the associated change in spray shape must be taken into account to avoid problems with spray impingement.