The autocatalytic thermal polymerization behavior of three benzoxazine monomers containing carboxylic acid functionalities is reported. Several mixtures of these carboxylic monomers and 3-phenyl-3,4-dihydro-2H-1,3-benzoxazine were prepared and their thermal polymerization behavior was analyzed by differential scanning calorimetry. The acid character of these reactive monomers increases the concentration of oxonium species, thus catalyzing the benzoxazine ring opening reaction. In this way the polymerization temperature decreased by as much as 100 degrees C in some cases. The existence of decarboxylation processes at high temperatures has been established by FTIR-ATR and related to the increase in thermal stability observed by TGA in some cases. A relationship between the presence of carboxylic groups in the resulting materials and their flame retardancy behavior has also been established. (C) 2008 Wiley Periodicals, Inc.