The effect of injection pressure on the structural transformation of flash-boiling sprays of gasoline and ethanol is investigated using a multi-hole injector in an optically accessible, single-cylinder spark-ignition direct-injection (SIDI) engine. The engine was run at 1200 revolutions per minute at wide-open-throttle conditions with no intake air boosting. In fully warmed-up engine environments, high-speed Mie-scattering imaging was performed for two fuel injection timings of 90 and 300 crank angle degrees after top dead centre, corresponding to low and high ambient pressure conditions, respectively. The injection pressure was also varied from 4 to 15 MPa. The macroscopic spray structure was quantified using spray tip penetrations, spray spreading angles and spray areas. From the measurements, it is found that fuel sprays injected at the earlier injection timing, when the vapor pressure of the fuel is higher than the ambient pressure, show the convergence of the spray plumes towards the injector axis evidencing the flash-boiling phenomenon. By contrast, fuel injected at the later timing, and hence, higher ambient pressure than the fuel vapor pressure, show typical spray structures with distinct plumes for each nozzle hole. The flash-boiling sprays appear to be influenced by the injection pressure significantly. It is observed that the convergence of spray plumes associated with the flash-boiling is less evident with increasing injection pressure, as higher injection momentum outperforms the plume-to-plume interaction resulting in increased directionality of the spray plumes. It is also found that the enhanced fuel vaporisation due to increased injection pressure is more prevalent in flash-boiling sprays than non-flash-boiling sprays. These findings suggest a great benefit of high pressure injection and flash-boiling sprays in achieving well-mixed charge in SIDI engines. Between gasoline and ethanol, the degree of flash-boiling appears to be different. It is observed that the gasoline sprays undergo flash-boiling and spray collapsing at the low ambient pressure condition while the transitional flash-boiling occurs at the high pressure condition. In comparison, the ethanol sprays experience the transitional flash-boiling at the low pressure condition and no flash-boiling at the high pressure condition. Also, the effect of injection pressure on the spray convergence and fuel vaporisation is more pronounced for the gasoline sprays, all because gasoline fuel has a greater degree of superheating than that of ethanol. (C) 2014 Elsevier Ltd. All rights reserved.