Molecular dynamics-free energy perturbation simulations are used to study the acidity of HCl relative to water at conditions ranging from ambient density and temperature to several supercritical water states. An increase in temperature from 25 degrees C to the critical temperature T-c (374 degrees C) along the coexistence curve reduces the difference between the pK(a) of HCl and pK(w) by a factor of 2. However, the corresponding difference in free energies of dissociation remains constant, even in supercritical water, along isochoric and isothermal (between 0.29 and 0.087 g/cm(3)) paths. This new observation is explained in terms of the effect of temperature on the thermodynamic properties of solvation of the various reacting species and is used to gain further insight into the behavior of previous experimental data.