This work contemplates the use of pristine multiwalled carbon nanotubes (MWNTs) as nanofillers in the preparation of bisphenol A-based polybenzoxazine and diphenolic acid derived polybenzoxazine. These materials were prepared by using a solventless method varying the MWNT amount from 0.1 to 1.0 wt%. MWNTs were found to disperse well within both benzoxazine monomers and the dispersion also appears to be good in the cured system. Rheological and electrical percolation thresholds were obtained for MWNT concentrations lower than 0.1 wt% indicating the existence of good affinity between MWNTs and polybenzoxazine matrices. The characterization of the resulting nanocomposites revealed that MWNTs affected polybenzoxazines differently. The limiting oxygen index of the nanocomposites increased as a function of the nanotube content, from 35.2 to 38.7 for a bisphenol A-benzoxazine based system (BPA-PBz) and from 31.2 to 37.4 for a methyl-4,4′-bis-[6-(3-phenyl-3,4-dihydro-2H-1,3-benzoxazine)]pentanoate based system (MDP-PBz), respectively. Moreover, MWNTs positively influenced the thermo-mechanical, thermal and mechanical properties of the nanocomposites. The resulting attractive properties have been attributed to good interaction between the polybenzoxazines and the finely dispersed nanofillers.