A wind tunnel test campaign has helped to identify the cause of the stagnation heat flux augmentation observed under high-enthalpy and high pressure conditions in several major shock tunnels. The test was carried out in the free-piston high-enthalpy shock tunnel HIEST under stagnation enthalpies from H0 = 8 MJ/kg to H0 = 20 MJ/kg with a stagnation pressure of P0 = 50 MPa. A flat plate with miniature coaxial thermocouples was used for surface heat flux measurements. To measure radiation and convection heating independently, optical filter windows passing different wavelengths were placed in the front of the thermocouples. There was significant radiation heating from wavelengths ranging from visible to near infrared, which supplied approximately 40% of the total heating. It was concluded that this radiation was a major cause of the heat flux augmentation. Comparison of heat flux records with and without optical filters showed that the heat flux increase was significantly delayed by the filter windows. Sequential Schlieren images showed that the delay was consistent with the establishment time of the shock layer, indicating that the radiation was mainly emitted from the shock layer.