Hydroquinone crystals have been found to undergo first-order phase transition to a denser polymorph with an enthalpy increment, as does resorcinol. The phase stable at room temperature (called alpha) transforms to the new phase (designated delta) only under elevated pressures. P-V-T relations of the new phase have been measured, and characteristics of the transitions and the triple point among the alpha phase, the delta phase, and the liquid have been estimated from the thermodynamic consistency. The thermal expansion coefficient, the internal pressure, and the heat capacities of the delta phase are significantly larger than those of the alpha phase. Rigid-body vibrations and/or librations may play an important role in the alpha delta transition. Contributions of hydrogen bonds and the structure of the delta phase are discussed.