The isobaric acid/base equilibrium between acridine and the acridinium cation was measured from ambient temperature to 380 degrees C (above the critical temperature of water, T-c = 374 degrees C) using absorption spectroscopy. At 3500 psia, the isobaric protonation of acridine is shown to be exothermic up to approximately 315 degrees C. Above 315 degrees C, protonation becomes endothermic due to changes in the dielectric constant of water with temperature, which has a profound influence on the solvation of ions. The results are interpreted using a modified Born equation to account for the temperature-dependent changes in acridinium cation and proton solvation. The absorption and fluorescence spectra and the fluorescence Lifetime of acridine are sensitive to changes in solvent-solute hydrogen bonding. Hydrogen bonding between acridine and water is observed to decrease from ambient temperature to the critical temperature. A relatively rapid change in hydrogen bonding occurs between 100 and 200 degrees C.