The miscibility of poly(methacrylic acid) (PMAA) and poly(vinyl acetate) (PVAc) blends was investigated by the CPMAS C-13 NMR. The H-1 spin-lattice relaxation time in the laboratory frame (T-1(H)) showed that the PMAA/PVAc blends are homogeneous on a scale of 20-50 nm, and the H-1 relaxation time, in the rotating frame (TH1rhoH) revealed that the PMAA/PVAc-rich blends are partially inhomogeneous on a scale of 2-5 nm, while the PMAA-rich/PVAc blends, including the PMAA/PVAc = 1/1 blend, are completely miscible on the scale. The domain size of the PMAA/PVAc = 1/1 blend was also estimated from the Goldman-Shen experiment. The obtained maximum domain size of 2.5 nm is consistent with the results of T-1rho(H). The decomposition of C-13 NMR spectra for the carboxyl (COOH) and carbonyl (COO) regions and the C-13 spin-lattice relaxation time in the laboratory frame (T-1(C)) suggested the. existence of an interpolymer interaction between PMAA and PVAc: the interaction is most likely a hydrogen bond and acts an important role for a great miscibility. The dependence of T-1(C) on the PMAA molar ratio indicates that the molecular motion of PVAc is largely affected by PMAA.