The choice of reaction progress variable on the statistical behaviour of the surface density function (SDF) and the strain rates, which govern the evolution of SDF, have been analysed using a detailed chemistry Direct Numerical Simulation (DNS) database of freely propagating statistically planar H 2 -air flames with an equivalence ratio of 0.7. The DNS database consists of three cases spanning the corrugated flamelets, thin reaction zones and broken reaction zones regimes of premixed turbulent

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... ombustion. For this analysis, the reaction progress variable is defined based on the mass fractions of H2, O2 and H2O. The mean variations of the SDF and the flame displacement speed have also been found to be dependent on the choice of the reaction progress variable. The progressive weakening (strengthening) of the preferential alignment of the reaction progress variable gradient with the most extensive (compressive) principal strain rate with increasing Karlovitz number leads to changes in the behaviours of normal and tangential strain rates from one combustion regime to another. The differences in displacement speed statistics for different choices of reaction progress variable affect the behaviours of the normal strain rate due to flame propagation and curvature stretch. The relative thickening/thinning of the reaction layers of the major species has been explained in terms of the statistics of effective normal strain rate experienced by the -isosurfaces for different choices of reaction progress variable.