A new blowdown wind tunnel facility is used to study reacting and non-reacting compressible, turbulent shear layers. In the reacting experiments reported here, low concentrations of hydrogen, fluorine and nitric oxide are carried by inert diluent gases, with the hydrogen and nitric oxide in the high-speed stream, and fluorine in the low-speed stream. The heat release associated with the resulting chemical reaction serves as a diagnostic for molecular mixing in the shear layer. This molecular mixing has been quantified at two compressibility levels by conducting 'flip' experiments in this new facility. The results indicate that the volume fraction of mixed fluid in the compressible shear layer is substantially reduced as compared to previous incompressible results. These same flip experiments also reveal that compressibility significantly reduces the entrainment ratio as compared to theoretical predictions extrapolated from incompressible models.