Increasing the compression ratio of a simple-cycle engine beyond 100:1 can enable first law efficiency approaching 60% if losses relative to the ideal cycle can be maintained similar to existing engines. Achieving this in practice will require understanding and managing the combustion process in the unique environment that results from such a high compression ratio. As a first step, this paper examines the behavior of combusting and noncombusting single-plume diesel sprays in a free-piston, extreme-compression device for compression ratios ranging from 30:1 to 100:1. High-speed schlieren photography through full bore optical access in the end wall of the combustor provides imaging of the spray. Spray penetration and dispersion (via spray angle) are measured as functions of time for each compression ratio, for both nonreacting and reacting sprays. For the combusting-spray experiments, ignition delay based on OH luminosity is also recorded via a high-sensitivity photodiode. A separate set of experiments using a diffuse back-illumination method provides measurements of penetration of the liquid phase spray region as a function of time and compression ratio. The results of this study are shown to be in general agreement with the current understanding of spray behavior as a function of parameters such as gas density, even when extended to this new regime. However, the steep free-piston motion profile results in density conditions varying greatly during the course of injection at the higher compression ratios, which in turn affects the spray penetration and spreading. Furthermore, a significant effect of in-cylinder fluid motion on the spray is observed. This effect becomes stronger at the higher compression ratios, likely due to the ratio of ambient density to spray density becoming low.