The mixing field in a supersonic cold flow past a set of compression-expansion ramps mounted at the downstream end of a strut, with staged injection at two tandem locations on the strut upstream of the ramps, has been experimentally investigated. Air and helium have been used to inject into a primary stream of air at an inlet Mach number of 2.0. Two test section geometries, one having a flow cross section twice that of the other, have been considered. The larger test section has wall struts as well as a middle strut, whereas the smaller one has the strut only in the middle. Terminal bulk pressure and wall static pressure measurements, and Schlieren visualization have been performed. The Mie scattering technique has been used to obtain the concentration contours of the injectant across the flow cross section at several axial locations downstream of the injection points. The results indicate better mixing with helium injection and less influence of the boundary layer along the test section walls with the larger test section, as expected. ©ASCE.