Direct numerical simulations (DNS) of temporally evolving weakly compressible shear layers with gradients of species and temperature have been performed to investigate the influence of detailed diffusion processes on turbulent mixing. As an exact modeling of the mixing process is a prerequisite of correct combustion prediction, this study aims at exploring the necessity of considering detailed diffusion processes of active scalars. The results comprise a demonstration of the strength of the Soret and Dufour effect and of their impact on the scalar and temperature gradients. The erroneous assumption of a spatially and temporally constant Schmidt number, equal for all species, is discussed. Furthermore, the scalar dissipation rate which is responsible for the smoothening of scalar fluctuations in the smallest scales is investigated. As this quantity is controlling the molecular mixing, it is clear that an influence on it by detailed diffusion processes has a direct impact on the chemical reaction and heat release rate in turbulent combustion.