Experimental results from a series of injection tests of pressurized H2, N2 gases into Mach 1.8 airfiows between parallel channel walls through a flush-mounted circular sonic opening have been presented.Schlieren pictures revealed complex interaction flow features including the occurrence of bow/separation shock waves due to the injection as well as the barrel shock/Mach disc structure inside the injected gas stream. The injectant penetration measured by the Mach disc height against the injection pressure showed a good agreement with the correlation curve based upon the “effective back pressure” concept. The reversed flow region beneath the separation shock wave, the injectant wake and its associated flow entraimment were also visualized by the oil paint method. Wall static pressure distributions around the injector were measured in detail, which corresponded very well to the above results of flow visualization.Gas samplings were also undertaken by using the pressure taps to confirm the presence of H2 gas in the separation region ahead of the injector. Traversing of total pressure and H2 gas concentration at the exit of the test channel showed monotonous increase of the loss while its profile was kept very similar with the injection pressure. The area indicating the loss and the presence of H2 gas almost coincided with each other, which remained to be small to indicate very slow gas mixing/diffusion with the main air flow. With the increase of airflow total temperature to 1200 K, a bulk flame was first observed at the exit section. Further increase up to 1460 K observed an ignition flame at the injector. However,the reflection of the bow shock wave was found to be a more likely trigger of the bulk flame ignition within the test section. Experimental results from a series of injection tests of pressurized H2, N2 gases into Mach 1.8 airfiows between parallel channel walls through a flush-mounted circular sonic opening have been presented. Schlieren pictures revealed complex interaction flow features including the occurrence of bow / separation shock waves due to the injection as well as the barrel shock / Mach disc structure inside the injected gas stream. The injectant penetration measured by the Mach disc height against the injection pressure showed a good agreement with the correlation curve based upon the “effective back pressure ”The concept of the static pressure distributions around the injector was measured in detail, which showed very good to the above results of flow visualization. Gas samplings were also carried by using the pressure taps to confirm the presence of H2 gas in the separation region ahead of the injector. Traverse of total pressure and H2 gas concentration at the exit of the test channel showed monotonous increase of the loss while its profile was kept very similar with the injection pressure. the loss and the presence of H2 gas almost coincided with each other, which remained to be small to indicate very slow gas mixing / diffusion with the main air flow. With the increase of airflow total temperature to 1200 K, a bulk flame was first observed At the exit section. Further increase up to 1460 K observed an ignition flame at the injector. However, the reflection of the bow shock wave was found to be a more likely trigger of the bulk flame ignition within the test section.