Recent visualization of n-dodecane delivered from a diesel injector into environments above 60 bar and 900 K performed at Sandia National Laboratories suggested a reduction in surface tension as droplets and ligaments were no longer detectible via back-illuminated microscopy. In the current study, improvements in optical microscopy are implemented to overcome much of the optical distortion present at these harsh conditions, leading to greater measurement resolution. The measurements show that the classical atomization and vaporization processes do shift to one where surface tension forces diminish with increasing pressure and temperature. Key indicators of “miscible mixing” include a deformation of liquid structure under minimal shear from surrounding gas velocities as well as indications that both large-scale turbulent motions and local molecular diffusivity simultaneously drive the mixing be-tween fluids of different densities. A new fundamental finding of this study is that the transition to miscible mixing does not occur instantaneously when the fluid enters the chamber at a given temperature and pressure where miscible mixing is observed. Rather, the large, cool liquid structure that was just injected exhibits surface tension at first, and then, after time surrounded by the hot ambient and other fuel vapor, undergoes a transition to miscible mixing.