The effect of tacticity on the thermal degradation behavior under nitrogen of poly(methacrylic acid)s has been investigated. The structural changes occurring in the polymer chains during the first of the two stages of decomposition, at temperatures lower than 300 degrees C, were followed by FTIR and NMR. In this stage, the degradation of highly isotactic poly(methacrylic acid)s essentially consists of the formation of sequences of six-membered cyclic anhydrides bg intramolecular dehydration. This process is likely to be favored by local conformations in which adjacent acid groups are fairly close. The presence of an increasing amount of syndiotactic units hinders the formation of cyclic anhydrides and shifts the decomposition at higher temperatures, making other reactions competitive, such as decarboxylation, formation of double bonds, and cross-linking. As a result, weight losses, solubility, and glass-transition temperatures of residues after the first degradation stage are proportional to their syndiotacticity.