The synergism in the glass-transition temperature (T-g) of ternary systems based on benzoxazine (B), epoxy (E), and phenolic (P) resins is reported. The systems show the maximum T-g up to about 180 degrees C in BEP541 (B/E/P = 5/4/1). Adding a small fraction of phenolic resin enhances the crosslink density and, therefore, the T-g in the copolymers of benzoxazine and epoxy resins. To obtain the ultimate T-g in the ternary systems, 6-10 wt % phenolic resin is needed. The molecular rigidity from benzoxazine and the improved crosslink density from epoxy contribute to the synergistic behavior. The mechanical relaxation spectra of the fully cured ternary systems in a temperature range of -140 to 350 degrees C show four types of relaxation transitions: gamma transition at -80 to -60 degrees C, beta transition at 60-80 degrees C, alpha(1) transition at 135-190 degrees C, and alpha(2) transition at 290-300 degrees C, The partially cured specimens show an additional loss peak that is frequency-independent as a result of the further curing process of the materials. The ternary systems have a potential use as electronic packaging molding compounds as well as other highly filled systems.