Forward recoil spectrometry was used to obtain the tracer (D*) and mutual ((D) over tilde) diffusion coefficients in a miscible polymer blend of poly(methyl methacrylate) (PMMA, T-g = 136 degrees C) and poly(styrene-co-acrylonitrile) with similar to 23 wt% acrylonitrile content (SAN, T-g = 112 degrees C). For blends with SAN weight fraction w of 0.2 and 0.5, the temperature dependence of D* for both species was nearly identical, Tracer diffusion coefficients D*(PMMA) and D*(SAN), were determined for matrices consisting of 176 000 molecular weight SAN (w=0.5) and various molecular weights of PMMA ranging from 27 000 to 840 000. The results were consistent with those expected from the mechanisms of reptation and constraint release. Analysis of the tracer diffusion coefficients D* showed that SAN has a monomer friction coefficient, zeta(0,PMMA), about five times smaller than that of PMMA (zeta(0,PMMA)) in a matrix of pure PMMA, but the difference decreased monotonically as w increased, so that the zeta(0),values were nearly equal when w=1. Corresponding to this relative change in zeta(0), the glass transition process is broad for PMMA-rich blends and narrow for SAN-rich ones, raising the possibility that the difference in zeta(0), in the PMMA-rich blends is due to the existence of two local glass transitions, one for each species in the blend. For blends at the composition of w = 0.5, the (D) over tilde was measured as a function of PMMA molecular weight. The data followed closely the predictions of the fast theory, the result expected if (D) over tilde is ultimately controlled by the diffusion of the faster moving species. From the measurements of (D) over tilde and D* at 187 degrees C, the composition dependence of the Flory interaction parameter was also obtained, which showed good agreement with the recent small angle neutron scattering results by Hahn et al.