Results are presented of direct numerical simulations of parallel/series reactions A + B --> R, R + B --> S in homogeneous turbulent flows. Both constant-rate and temperature-dependent reactions are considered. The results of simulations with constant-rate reactions indicate that the initial reactants＇ conditions, the molecular diffusion and the differential diffusion have significant influence on the low-order moments of the scalars and on the chemical selectivity (X-sel). For moderate to fast reactions, the selectivity is characterized by the initial size of the reactants and is improved (X-sel decreases) as the average size of the initial reactant length scales decreases. The results of simulations conducted with temperature-dependent reactions indicate that the selectivity and the statistics of the scalars and temperature are strongly dependent on the initial temperature conditions. The magnitudes of X-sel are considerably higher and the temperature fluctuations decay faster when the temperature is initially correlated with the reactant B rather than A. The correlations between the temperature and the reactants and the effect of the initial temperature field on the selectivity decreases as the magnitude of the thermal diffusivity and the amount of heat release increases, or the activation energy (Zeldovich number) decreases.