The differing glass transitions between deuterated polystyrene and poly(alpha-methylstyrene) are shown to have a profound effect on their interdiffusion coefficient. We have measured the interdiffusion coefficient as a function of concentration and temperature for two blends. The results can be interpreted in terms of the ＇fast＇ theory of interdiffusion with the temperature dependence obtained from a Williams-Landel-Ferry (WLF) treatment. The intradiffusion coefficients of the two polymers have also been deduced by this analysis. It is found that the deuterated polystyrene has a much more concentration-dependent intradiffusion coefficient than the poly(alpha-methylstyrene). Thermodynamic slowing down is also observed in one blend.