The photoinitiated ring-opening cationic polymerization of a monofunctional benzoxazine, 3-phenyl-3,4-dihydro-2H-1,3-benzoxazine, with onium salts such as diphenyliodonium hexafluorophosphate and triphenylsulfonium hexafluorophosphate as initiators was examined. The structures of the polymers thus formed were complex and related to the ring-opening process of the protonated monomer either at the oxygen or nitrogen atoms. The phenolic mechanism also contributed, but its influence decreased with decreasing monomer concentration. Thermal properties of the polymers were also investigated by differential scanning calorimetry and thermogravimetric analysis. (C) 2003 Wiley Periodicals, Inc.