The present work reports the results of a priori tests of Reynolds-Averaged Navier–Stokes (RANS) models based on Direct Numerical Simulations (DNS) data of zero-pressure-gradient flat-plate turbulent boundary layers. The DNS database covers a wide range of thermodynamic operating flow conditions, from supersonic (M∞ = 2.25) up to the high-enthalpy hypersonic regime (M ∞ = 12.48). The most common RANS closures and compressibility corrections in literature are assessed against the exact terms from the DNS. Particular attention has been paid to closure models for the turbulent heat fluxes and the dilatational dissipation, as well as to the analysis of turbulence/chemistry and turbulence/vibrational relaxation interactions for the high-enthalpy simulations.