The variation of the acid dissociation equilibrium of trifluoromethane sulfonic acid in dilute aqueous solution along the liquid-vapor dissociation curve is treated by a dielectric hydration model. The computational results indicate that the dissociation equilibrium becomes unfavorable from the point of view of the free energy change as temperature is increased above 500 K where the solvent density is less than 85% of the triple-point density. These results give additional perspective for the view that insufficient hydration can result in incomplete proton dissociation from sulfonic acid headgroups in Nafion membranes.