We report observations regarding changes in surface morphology for transient fuel jets as the ambient conditions approach the critical properties of pure fuels. Both ballistic imaging and ultrafast shadow imaging were applied to four fuels as they were injected through a single hole Diesel injector into a spray research chamber operated at three different ambient conditions that span the range of critical properties for the pure fuels that were studied. The results indicate that the pure fuels (butanol, dodecane, and hexadecane) tend to undergo a change in image structure that usually scales with estimated mixture critical properties. Commercially available Diesel fuel is not strongly affected, even at the highest pressure and temperature conditions. (C) 2015 Elsevier Ltd. All rights reserved.