Ortho-, meta-, and para-phenylnitrile-functional benzoxazines are polymerized at different compositions with phthalonitrile-functional monomers providing copolybenzoxazines of high thermal stability and easy processability. The most positive effect on these properties is obtained on copolymerizing phthalonitrile- and ortho-phenylnitrile-functional benzoxazines. Thermal decomposition of these polybenzoxazines is studied by thermogravimetry (TGA). It is demonstrated that only 30 mol % of phthalonitrile-functional benzoxazine added to the ortho-phenylnitrile-substituted monomer improves char yield significantly from 59 to 77 wt %, which is the value of neat phthalonitrile-based polybenzoxazine. Glass transition temperature (T-g) also dramatically increases from 180 degrees C for neat ortho-phenylnitrile polymer to 294 degrees C for the copolymer with 30 mol % of phthalonitrile-functional monomer. Additionally, the high melt viscosity of phthalonitrile-functional benzoxazines is dramatically decreased upon blending with phenylnitrile-functional monomer.