The interdiffusion of deuterated polystyrene (d-PS) and poly(methyl methacrylate) (PMMA) of molecular weights low enough to enable total miscibility is studied at temperatures slightly above the glass transitions of both components. From diffusion profiles obtained by nuclear reaction analysis a function is derived which accurately describes the observed concentration dependence of the mutual diffusion coefficient. This function is compared to the predictions of the "slow" and "fast" theories of diffusion and is found to describe a thermal transition region between the two theories. Tracer diffusion coefficients are seen to vary with temperature according to the WLF equation but do not scale according to the Rouse theory. This is probably due to the narrow ranges of polymer molecular weights and temperatures studied.