Modified novolac resins with benzoxazine rings were prepared and cured with isobutyl bis(glycidylpropylether) phosphine oxide (IHPOGly) as crosslinking agent. Their curing behaviour using different epoxy/phenol molar ratios and with or without triphenylphosphine as catalyst was studied. Two different phenolic groups react with oxirane ring, those initially free and those generated after benzoxazine ring opening. In absence of catalyst, it is not possible to distinguish between them. However, for the catalyzed curing of the highest modification degree benzoxazine based novolac resin is possible to distinguish both reactions. The thermal, thermomechanical and flame retardant properties of the cured materials were measured. V-O materials were obtained when the resins were tested for ignition resistance with the UL-94 test. (C) 2004 Elsevier Ltd. All rights reserved.