The miscibility and phase structures of initially miscible novolac/poly(methyl methacrylate) (PMMA) blends after full curing of the novolac are investigated by high-resolution solid-state NMR, FT-IR, and DSC techniques. The strong intermolecular hydrogen-bonding interaction between novolac and PMMA results in the blends being miscible at the molecular level, and the phase-separation process of the blends is very slow even at 200 degrees C. Curing novolac/PMMA blends with hexamine reduces the intermolecular hydrogen bonding significantly, but a considerable amount of residual intermolecular hydrogen bonds still remains in the cured blends. The phase structures of the fully cured novolac/PMMA blends show composition dependence. Fully cured novolac-rich blends result in semi-interpenetrating networks; novolac forms a highly cross-linked network through the whole blend while PMMA chains distribute uniformly in the network with domain sizes on 2-3 nm scales. The segmental motion of the PMMA chains is frozen by the network, and the glass transition is not observed. When PMMA is rich, cross-linked novolac chains distribute in the PMMA matrix. Most fully cured blends are partially miscible or microphase-separated systems with domain sizes around a scale of 20-30 nm. The phase structure and properties of phenolic resins can be modified and extended by blending novolac with a high polymer with careful choice of the blending composition and curing conditions.