The weak thermal development of a reacting high speed high enthalpy hydrogen-air mixing layer which seems to depart from equilibrium only slightly in terms of reaction rates is analysed through the examination of stretch, and details of chemistry from the direct numerical simulations of the reacting mixing layer with full chemistry. The analysis of these parameters clearly indicates that the peak temperatures and the local composition depart from equilibrium significantly consistent with the high stretch even though the departures of the reaction rates from equilibrium apparently are not large. A serious consideration of chemical non-equilibrium effects in high speed mixing layers is called for.