The pH of aqueous sulfuric acid and sulfuric acid-ammonia mixtures was measured using the optical indicator acridine at temperatures from 200 to 400 degrees C and pressures from 3500 (24.1) to 6000 psia (41.3 MPa). Because of large changes in the pK(a) of protonated acridine in supercritical water (SCW), the measurable pH range shifts from 2-4 at a density of 0.60 g/cm(3) to 4.5-7 at a density of 0.24 g/cm(3). At 3500 psia, the first dissociation constant (K-a1) of H2SO4 decreases sharply with increasing temperature above 350 degrees C, primarily due to a reduction in density and thus the solvation of the bisulfate and hydrogen ions. The acidity of H2SO4 relative to HCl increases with increasing temperature at constant pressure up to the critical point of pure water. Based on titrations of sulfuric acid solutions with ammonia, weak acid-weak base behavior is observed at 380 degrees C and 5000 psia (34.5 MPa). At these conditions the system H2SO4-NH4-HSO4 may be used as a buffer to maintain pH in the range 3.5 +/- 0.25.