The heat flux and boundary layers over a flat plate at suborbital and superorbital speeds were measured in a freepiston expansion tube at enthalpies of 26.0 and 53.4 MJ/kg, respectively. Estimates of the gas-phase and surface Damkohler numbers were made that indicate that although the boundary layer might be frozen, there is a possibility of surface reactions occurring. The heat-flux results were compared with theoretical predictions of heat flux for both frozen and equilibrium chemistries. The results indicated that the influence of real gas effects, such as recombination and surface catalycity, were minimal for the present flow conditions. Interferograms were obtained using resonance enhancement of the thermal boundary layer over a flat plate. Comparison of the measured boundary-layer thickness with several expressions used to predict the boundary-layer thickness allowed an evaluation of their effectiveness at high enthalpies.