This paper presents and discusses Implicit Large Eddy Simulation (ILES) results of vitiation effects in ground tests of supersonic air/H2 combustion. This work is useful for realistic extrapolation of ground test data to flight conditions. The high-enthalpy flow configuration retained, typical of scramjet engines, is the Mach 2 LAERTE combustion chamber of the French aerospace lab ONERA. The supersonic air co-flow is preheated by burning a small amount of hydrogen in oxygen-enriched air. Stable numerical simulations of such high-speed turbulent shocked flows require dissipative numerics that interact with molecular diffusion. The premature ignition in the case of vitiation by oxygen atom can be observed using reduced finite rate chemistry in the quasi-laminar approximation, i.e. without any subgrid combustion model, because the Damköhler number (ratio of turbulent to chemical time scale) is less than unity. Turbulent time scales are only moderately affected by vitiation, but chemical time scales are significantly reduced.