NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. Experiments have been conducted in the GALCIT Supersonic Shear Layer Facility to investigate some aspects of mass injection in subsonic diffusers. The goal of the experiment is to study aerodynamic control in subsonic diffusers by investigating downstream velocity profiles. These experiments were designed to address several key issues like effects due to velocity change (for one-stream and two-stream flows), and effects due to density variation. The effect of the separation bubble (stall flow) on the performance of the diffuser has also been investigated. One-stream experiments were performed with non-reacting (cold) runs using [...] in the high-speed section at different velocities and zero velocity at the low-speed section. Detailed analysis of data obtained shows a slight dependency of the reattachment point of the separation bubble on Reynolds number. As the flow rate in the high-speed section increases, the reattachment point of the separation bubble shifted slightly downstream. Two-stream flow (low- and high-speed sections) experiments were performed using [...] in the high-speed section and a density-matched mixture of Argon/Helium in the low-speed section. As the mass injection is increased in the low-speed section, the reattachment point of the separation bubble moved further downstream. While keeping the same velocity ratio (low- and high-speed sections), as the overall flow velocity increases, the reattachment point moved further downstream. Also, experiments with a higher density ratio using Argon in the low-speed section and [...] in the high-speed section were performed. As the density of the low-speed section increases, the reattachment point of the separation bubble moved upstream due to shear-layer entrainment effect. Finally, as the reattachment point of the separation bubble shifted further downstream, the diffuser pressure coefficient decreases, therefore, the performance of the diffuser is degraded. Schlieren flow visualization and pressure probes were used in the experiments.