Mixing in turbulent reacting flows is well described within the framework of iso-scalar surfaces. Scalar-gradients are normal to their corresponding iso-surfaces and their transport equations explicitly display contributions of small-scale turbulent straining, molecular diffusion and chemical conversion [1]. Two popular closures in premixed turbulent combustion use transport equations for the scalar fluctuation dissipation rate (SDR) and the flame surface density (FSD), related to the scalar-gradient magnitude and to the normal distance between adjacent iso-surfaces, respectively [2]. The transport equation for the gradient of the reaction progress variable includes generation/destruction of scalar-gradient magnitude by the small-scale turbulent normal strain rate, reaction and molecular diffusion, quantified as characteristic times for these processes. It is conjectured that in the computational domain there exist either reaction- or strain- controlled zones. A Direct Numerical Simulation (DNS) database of statistically planar turbulent premixed flames for various Karlovitz numbers, ranging from the wrinkled/corrugated flamelets to the thin reaction zone regime in the combustion diagram, is used to compute the statistical behaviors of flame normal strain, reaction and molecular diffusion rates. Normal strain is apparently of the order of the Kolmogorov strain rate. The existence of reactive-diffusive zones, with comparable chemical and diffusion times, is apparent, while that of convective-diffusive zones, with Kolmogorov time micro-scales of the same order of the diffusion times, is less obvious. It may be concluded that small-scale turbulence interacts weakly with the chemistry in the reaction zone, acting only in the preheat zone. A scalar/scalar-gradient magnitude joint PDF transport equation is derived. A MonteCarlo simulation, based on a numerical discretization of molecular diffusion terms, is proposed and preliminary results are compared with a DNS of one scalar with an Arrehnius reaction in statistically homogeneous and stationary turbulence of a constant density fluid. REFERENCES [1] C. Dopazo and L. Cifuentes. The Physics of Scalar Gradients in Turbulent Premixed Combustion and its Relevance to Modeling, Combust. Sci. Tech. 188, 1376-1397, 2016. [2] C. Dopazo, N. Swaminathan, L. Cifuentes and X.S. Bai. Premixed Combustion Modeling, Ch. 3, 100-161, in Recent Advances in Combustion, Eds. N. Swaminathan et al., CPU, 2021.