Site-directed isotopic enrichment and solid-state C-13 spin-diffusion NMR techniques were employed to characterize miscibility at the molecular level and phase separation in blends of amorphous polymers. Using 2D proton-driven spin-diffusion techniques on a mixture of C-13-labeled poly(styrene-co-maleic anhydride), [(CH2)-C-13,(CO)-C-13]SMA (27 wt % MA, M-w similar to 4 x 10(4) g mol(-1)), and poly(styrene-co-acrylonitrile), [(CN)-C-13]SAN (27 wt % ACN, M-w = 1.3 x 10(5) g mol(-1)), it is demonstrated that SMA and SAN are miscible on the molecular level. However, a specific orientation of the nitrile moieties in SAN with respect to the carbonyl groups in SMA is unlikely. This suggests that the miscibility of SAN and SMA copolymers is associated mainly with a decrease of intramolecular repulsion upon mixing, as opposed to a specific exothermic binary interaction. In addition, spin-diffusion experiments show that upon selective cross-linking of the SMA phase semi-interpenetrating networks are formed. Increasing the degree of cross-linking results in increasing degrees of phase separation.