A kinetic characterization of the excited-state deprotonation of beta-naphthol in sub- and supercritical water establishes large deviations from Arrhenius behavior above 110 degrees C. The fluorescence decay rate constant increases much less with temperature than expected and then decreases at temperatures and pressures beyond the critical point. The loss of solvation of water above 200 degrees C strongly inhibits its ability to accept a proton and accelerates the reprotonation rate. Under basic conditions, where water clustering and reprotonation are much less important, the observed decay rates deviate much less significantly from the expected Arrhenius behavior. Above the critical point, the relaxation rate constants exhibit strong pressure dependence and are linearly related to the square of the solution density. In contrast with the absence of shifts in the absorption spectrum, pressure-induced shifts in the observed emission maxima near the critical region are assigned to the involvement of contact ion pairs derived from partially deprotonated molecules, and emission from the excited state of the naphtholate-potassium [NapO(-)-K+] ion pair is observed in KOH solution at 200 degrees C.