Eckert and Weise first measured time-averaged energy separation behind a circular cylinder in 1943- Although a significant amount of work has been carried out on the subject since Eckert and Weise's discovery no time-resolved measurements have been made of the phenomenon at high subsonic Mach numbers. The aim of this project was to make these measurements, along with those of surface pressure. Energy separation and base pressure are investigated at high subsonic Mach numbers, behind a circular cylinder in crossflow, for the first time.;The measurement of energy separation has involved developing a novel operating procedure for a high frequency response thin film total temperature probe, allowing it to be heated in stagnant conditions while keeping the wind tunnel running, hi addition the analysis of the results has involved the development of a fully automated phase lock averaging routine.;The principal original contribution of this work is to demonstrate clearly that unsteady energy separation occurs as a result of vortex shedding at high subsonic Mach numbers. The time-resolved measurements show how the areas of increased and decreased total temperature and total pressure are related. The results also give, a good qualitative description of the shape of the vortex street, showing the presence of interconnecting ribs and the areas of maximum entropy increase and thus drag creation.;The surface pressure results have permitted the study of how Strouhal number, drag, base drag and vortex shedding mechanisms change with Mach number. Of particular note they show that the vortex mechanism present in the permanent shock wave regime does not replace that present in the intermediate shock wave regime but rather develops from it.