The UV-visible spectra of aqueous o-, m-, and p-nitrophenol were measured as a function of pH at temperatures from 50°C to 225°C at a pressure of 7 MPa. These were used to determine equilibrium constants for the acid ionization reaction of each isomer. The new experimental results, along with data from the literature, were used to develop a thermodynamic model to describe the dependence of ionization properties on temperature and pressure. The model yields predictions of the ionization constants for o-, m-, and pnitrophenol, log Ka, to at least 250°C and 20 MPa with an estimated uncertainty in log Ka of less than ± 0.06. Thermal decomposition studies, carried out by using the system as a stopped-flow reactor, showed that the three isomers are significantly more stable in HCl acid solutions. At temperatures above 225 oC and alkaline solutions, the residence time of the solution in the cell and the pre-heater is so long that significant changes in the spectra of the nitrophenolate species are observed in stopped-flow experiments as an indication of thermal decomposition reactions.