The local structure of sodium ions in poly(ethylene-ranmethacrylic acid) ionomer neutralized by sodium hydroxide has been investigated using Na-23 MAS and multiple-quantummagic-angle-spinning (MQMAS) NMR techniques at a high magnetic field. Isotropic chemical shifts and quadrupolar coupling products of three kinds of nonequivalent Na-23 ions, i.e., isolated, hydrated, and aggregated Na-23 ions were estimated from peak positions in Na-23 MQMAS spectra. The MQMAS technique at high magnetic fields is expected to be a powerful tool to investigate the local structure in amorphous systems containing quadrupole nuclei.