Thermal dehydration process of PMAA was investigated by solid-state C-13 NMR. For heat-treated PMAA at 150 degrees C, at which the dehydration goes very slowly, we observed three C-13 peaks at 172, 178, and 187 ppm in the carboxyl group region. The peak at 172 ppm is due to the intramolecular cyclic anhydrides by comparing the reported value of C-13 chemical shift. The peaks at 178 and 187 ppm were assigned to regularly aligned free carboxylic acids and intermolecular acid dimers, respectively, from the 2D-exchange C-13 NMR spectra, C-13 chemical shift values and IR spectra. We concluded that by heat-treatment the rearrangement of intermolecular hydrogen bonding of the carboxylic acids in PMAA occurs firstly to form the regularly aligned acid dimers, and the dimers dissociated to be the regularly aligned free carboxylic acids at high temperatures. The adjacent free carboxyl acids dehydrate with each other, resulting in the formation of intramolecular anhydrides.