The thermal behavior of a series of polybenzoxazines based on aniline and various phenols is examined under both inert and oxidative environments. Under an inert environment, the various phenols substantially affect the char yield of polybenzoxazines without altering the mechanism of Mannich base cleavage. Under an oxidative environment, the various phenols have a significant influence on the degradation behavior of polybenzoxazines below 600 degrees C. The mechanism of the Mannich base cleavage under oxidative degradation has been proposed.