The effect of temperature on hydrogen bonding was investigated by measuring the integrated infrared absorbance of various hydrogen-bonding modes as a function of temperature. It was found that conformationally preferred hydrogen-bonding modes maintain constant intensities over the wide temperature ranges studied for both polybenzoxazines and a novolac-type phenolic resin. In particular, the O- - -H- - -N hydrogen bond shows strong bonding that does not change over the temperature range. On the other hand, statistically distributed hydrogen bonding is more sensitive to the temperature change and its infrared intensities start decreasing around the beta-transition of polybenzoxazines. The unusual physical and mechanical properties of polybenzoxazines, including volumetric expansion upon polymerization, high moduli, and high glass transition temperatures, are explained based on the complex hydrogen bonding.