Solid-state NMR can be used to study the interface in polymer composites and blends. Such data indicated that (g-aminopropyl)triethoxysilane coupling agent in polyamide 6-SiO2 composites reacted with the SiO2 surface to form a network and with the matrix polymer with formation of at least some chem. bonds. Interpenetrating network formation may also occur. NMR data for poly(vinylidene fluoride) (PVDF)-poly(methylmethacrylate) (PMMA) blends, generally considered compatible, the mixing is not so perfect on a mol. scale. New triple-resonance expts. show how the interface between PVDF and PMMA can be quant. studied.