Polymerization reactions of a new aldehyde-functional benzoxazine (4HBA-a) were investigated in detail. The curing behavior of 4HBA-a was studied by differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) methods. The results indicate that the disappearance of the aldehyde group from 4HBA-a and the ring-opening reaction of 4HBA-a occur simultaneously. Gases evolved during the curing process of 4HBA-a were analyzed by thermogravimetric analyzer interfaced with FTIR spectra. The elimination of CO2 is attributed to the oxidation and decarboxylation of the aldehyde groups. In addition, the crosslink sites of the aldehyde groups in the polymer structure are confirmed by model reactions. A possible reactive position should be sited in ortho position of phenol rather than ortho and/or para positions of N-phenyl ring. Finally, the crosslinked structures of polymerized 4HBA-a have been proposed. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 1671-1677, 2011