The equilibrium alpha beta transition temperature, T-alpha beta, between the alpha crystalline phase and the beta crystalline phase of resorcinol has been determined when both supercooling and superheating effects vanished. The latent volume and dT(alpha beta)/dP have been determined by use of a precision pycnometer, and the thermodynamic characteristics of the alpha beta transition are presented. The P-V-T relations of each phase have also been obtained. All of the thermal expansion coefficient, the isothermal compressibility, and the internal pressure for the higher-temperature, higher-density polymorph, the beta crystal, are much larger than those for the lower-temperature, lower-density polymorph, the a crystal. Contributions of the hydrogen bonds and the van der Waals energy to the internal energy have semiquantitatively been analyzed by use of a simple potential model. When the a crystal transforms to the beta crystal, the energy of hydrogen bonds decreases due to the breakdown of the hydrogen bonds, whereas the van der Waals energy increases with the contraction in volume. Since the latter cancels the most of the former, we observe the small latent heat. The breakdown of the hydrogen bonds induces a significant change in the potential depths of the covalent structure O-H ... O and ionic, proton-transferred structure O-... H-O+ in each hydrogen bond. In consequence, the distribution of the protons between the covalent structure and the ionic structure changes with the transition. The increase in the entropy produced from the redistribution of the protons is the same order of magnitude as the latent entropy.