  In order to understand the long known anomalies in the composition dependence
of diffusion and viscosity of binary mixtures, we introduce here two new models
and carry out extensive molecular dynamic simulations. In these models, the two
molecular species (A and B) have the same diameter and mass. In model I the
inter-species interaction is more attractive than that between the pure
components, while the reverse is true for model II. Simulations and also mode
coupling theory calculations reveal that the models can capture a wide variety
of behavior observed in experiments, most interesting among them are the
non-monotonic variation of diffusion and viscosity with the composition and the
re-entrant viscosity dependence of the relaxation time.
