The inter-polymer interaction, morphology and molecular motion of the poly(ethylene oxide)/poly(methacrylic acid) (PEO/PMAA) complex were investigated by measuring various nuclear magnetic resonance parameters, such as C-13 chemical shift, (HT1)-H-1, (HT1p)-H-1, (HT2)-H-1 and (CT2)-C-13. For the complex, we observed two peaks for the carboxyl carbon of PMAA. We assigned the higher-field resonance to the carboxyl group that forms hydrogen bonds to PEO (the complex form), and the lower-field one to the group that forms hydrogen bonds among PMAA (the dimeric form). It is shown that, for temperatures within 100 K below T-g, the complex form easily breaks up and rearranges to the dimeric form. For the complex, the (CT2)-C-13 and (HT2)-H-1 reveal that PEO is mobile, whereas PMAA is rigid. This different mobility between PEO and PMAA may facilitate breakage of the hydrogen bonding between PEO and PMAA. Examination of H-1 spin diffusion in the complex reveals that the distance between PEO and PMAA in the complex is similar to that between PEO and PMAA in the dimeric form. These results show that the PMAA in the dimeric form does not aggregate to form a domain structure, and that the PEO/PMAA complex is miscible on a segmental scale. Furthermore, the thermal degradation of PMAA in the complex was examined. Dehydration occurs in the dimeric form to produce anhydride, and the reaction temperature is much lower than that of pure PMAA.