The phenomena of binary mixing and reaction of scalars (A, B) with unequal molecular diffusion coefficients (Gamma(A),Gamma(B)) in turbulent flows are investigated by direct numerical simulation (DNS). Homogeneous turbulent flows are considered with and without the presence of constant mean scalar gradients, under both nonreacting and reacting nonpremixed conditions. The results indicate the significance of the differential diffusion (Gamma(A) not equal Gamma(B)) effects on the low-order moments of the differential diffusivity variable (Z), the normalized scalars＇ variance difference (zeta), the scalars＇ cross correlation coefficient(rho), and the moments of reacting scalars. It is suggested that the behavior of rho and zeta are approximately characterized by the differential diffusion parameters including Gamma(r) (the ratio of the diffusivities) and Td (the difference of the diffusivities), and by the reactant conversion parameters including Da (the Damkohler number), and (S) over cap c (the Schmidt number corresponding to the average of the diffusivities of the reactants). In the comparative assessment of the results for non-reacting flows or reacting flows with slow chemistry, rho and zeta are parameterized by the differential diffusion parameters. In comparisons of reacting flow results, the reactant conversion parameters characterize rho and zeta. The influence of differential diffusion on the scalar statistics is reduced by the presence of a constant mean scalar gradient and/or by the increase of the Reynolds number. The statistical results are modified by the overall stoichiometry of the mixture.