An investigation has been conducted to determine the effects of jet-to-free-stream dynamic pressure ratio on the penetration and mixing of a sonic hydrogen jet injected normal to a Mach 4 airstream. The hydrogen gas was injected from a circular nozzle flush mounted in a flat plate with a turbulent boundary-layer thickness of 2.70 injector exit diameters at the injector station. The investigation was conducted for values of the dynamic pressure ratio ranging from 0.5 to 1.5. At five downstream stations between 7 and 200 injector diameters the mixing region was surveyed to obtain hydrogen volume concentration and pressure profiles. Results of the investigation indicate that the thick boundary layer had significant effects on the penetration and maximum concentration trajectories when compared to data correlations from other sources. The penetration trajectory was found to be proportional to the dynamic pressure ratio raised to the 0.3 power. The decay of the maximum concentration was very rapid in the near field and inversely proportional to (x/dj)^0.8 at downstream distances greater than 30 jet diameters. At a particular value of x/d the maximum concentration was proportional to q_r^½. Nondimensional concentration profiles, represented by Gaussian-type functions, on the vertical centerline showed similarity at values of x/d equal to or greater than 60. / Master of Science