An experi ental investigation has been made at a Mach number of 2.21 on the effect of air injection in separated supersonic flows using two-dimensional backward facing step models. This effect, in a laminar flow, was found to be in- dependent of the particular technique of injecti n when the rate of injecti n was small. Air injection raises the base pressure and decrea es the pressure gradient at reattachment. At high rates of injection, considerable differences are found between the results obtained for various techniques of injection. It was also shown that the increase of the base-pressure is larger in the turbulent case than for a laminar boundarylayer. When freon gas was injected, it produced the opposite effect, namely of decreasing the base-pressure at low rates of injection. (Author).