The flowfield associated with tangential slot injection of a gas into a supersonic external stream is of current technological and academic interest. The present study can be divided into three areas. First, detailed profiles in the near flowfield were determined with an interferometer for air injection into a Mach 2.1 airstream past a flat plate. Second, an analytical treatment of the turbulent mixing zone including the effects of a splitter plate boundary layer is presented and shown to provide predictions in excellent agreement with the experiments. Lastly, the stability of the mixing region is considered and experiments with spark shadowgraphs are reported.