Hydrogen gas has been injected transversely into Mach 1.8 airflow between parallel channel walls from two circular sonic injectors which were mounted flush and placed in tandem along the centre line of the bottom wall plate of a test section. Both cold and hot airnow conditions, i.e. atmospheric and heated to max.1460K total temperature, were tested. In the cold flow experiments, detailed measurements were successful and the tandem injection resulted in a marked difference from the single injection with respect to the pattern of shock waves and the distribution of pressure and hydrogen concentration near the injector region. Upon changing the injection pressure ratio between the two injectors, it has been revealed that a prior injection upstream of the main injection would be beneficial in terms of the total pressure loss in the airflow and the hydrogen concentration near the injector region while maintaining the mixing performance. In general, the flow features showed little difference as the airflow temperature was raised until hydrogen burning was observed, whence the results became inevitably less quantitative. Against the expectation from the cold flow tests, a prior injection of cold hydrogen has resulted in quenching the main injector flame. Hydrogen gas has been injected transversely into Mach 1.8 airflow between parallel channel walls from two circular sonic injectors which were mounted flush and placed in tandem along the center line of the bottom wall plate of a test section. Both cold and hot airnow conditions, ie atmospheric and heated to max. 1460K total temperature, were tested. In the cold flow experiments, detailed measurements were successful and the tandem injection resulted in a marked difference from the single injection with respect to the pattern of shock waves and the distribution of pressure and hydrogen Since changing the injection pressure ratio between the two injectors, it has been revealed that a prior injection upstream of the main injection would be beneficial in terms of the total pressure loss in the airflow and the hydrogen concentration near the injector region while maintaining the mixing performance. In general, the flow features showed little differenc Against the expectation from the cold flow tests, a prior injection of cold hydrogen has resulted in quenching the main injector flame.