A new class of phenolic thermosetting resins is developed that is based on the ring-opening polymerization of a benzoxazine precursor. These new materials seek to combine the thermal properties and flame retardance of phenolics with the mechanical performance and molecular design flexibility of advanced epoxy systems. These materials overcome many of the traditional shortcomings of conventional novolac and resole-type phenolic resins, while retaining their benefits. The viscoelastic behavior of the polybenzoxazines during isothermal cure is monitored by dynamic mechanical analysis. Isochronic measurements show that although the aniline-based benzoxazine has a lower activation energy for the gelation process than the methylamine-based resin, it has a slower rate of reaction. The purified monomer and as-synthesized precursor for each benzoxazine are found to polymerize by the same mechanism, despite the absence of an initiating species in the purified resins. The chemical gelation phenomenon of the methylamine-based resin is probed by a multifrequency dynamic cure analysis that allows determination of the instant of chemical gelation, as well as the network relaxation exponent, n. The constant value of the exponent regardless of cure temperature demonstrates that chemical gelation is, in fact, an isoconversion event for the methylamine-based benzoxazine. The multifrequency and isochronic analyses are shown to produce very similar gel times and activation energies for the gelation process.