The chemical stability of typical polybenzoxazines based on bisphenol-A and primary amines was studied. Using proton nuclear magnetic resonance spectroscopic analysis of polybenzoxazine model dimers, it was found that the Mannich base is stable in a carboxylic acid solution. It is proposed that the nature of the primary amines is responsible for the interactions which take place between the carboxylic acid and the Mannich-base model dimers. As a result, the chemical stability of polybenzoxazines may also be related to the nature of the amines, which, in turn, influence the strength of the hydrogen-bonded network structure which develops upon cure.