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An experimental study was made of incident-shock test time and end- wall, reflected-shock pressure at test conditions characteristic of high- pressure, high-enthalpy shock tunnels operating in the tailored-interface mode. The driven gas in these experiments was either air or nitrogen and the driver gas was heated hydrogen. The incident-shock Mach number varied from 7.5 to 10.6 and the initial driven-tube pressure varied from 10 to 150 cm Hg. Measurements of the radiation intensity behind the incident shock and Flagg's interpretation of the end-wall 'pressure dip' were used to infer the arrival of the interface region at three different axial stations in the driven tube. Incident-shock test time, for both air and nitrogen, was found to be considerably less than Mirels' turbulent boundary-layer test-time theory predicts at the test conditions commonly used in high-pressure shock-tunnel work. Combustion between the driver and driven gas was observed for the combination of hydrogen and air but this had no measurable effect on the usable test time. Mirels' theory appears to underestimate the mass flow in the boundary layer, particularly at high Reynolds number test conditions. Suitable modifications of this theory are suggested.