Equilibrium total pressures have been measured above aqueous HNO3 (for 7.82, 15.73, and 35.99 mol kg(-1) solutions from 294.6 to 224.7 K) and aqueous HCl (9.45 and 10.51 mol kg(-1), from 289.4 to 199.5 K) using a capacitance manometer. Equilibrium partial pressures of the acids have also been determined, by mass spectrometry, from 274.8 to 234.6 K for both HCl solutions, and from 265.0 to 240.1 K for 15.73 mol kg(-1) HNO3. Results are generally consistent with model predictions, though with small (similar to 10%) systematic deviations for the total pressure measurements over aqueous HCl at about 220 K. Mixtures of HCl-HNO3-H2O composition yielded measured total pressures orders of magnitude greater than predicted for the gases H2O, HNO3, and HCl. Mass spectrometric determinations and equilibrium thermodynamic calculations suggest that Cl-2 and NOCl were produced by the reaction: 4H((aq))(+) + NO3 (aq)- + 3Cl((aq))(-) reversible arrow NOCl(aq) + Cl-2(aq) + 2H(2)O((1)), which is known to occur in aqua regia (a mixture of concentrated hydrochloric and nitric acids). Calculations for aqueous solutions of stratospheric aerosol composition suggest, purely on equilibrium grounds (and neglecting kinetics), that the reaction could be a source of active chlorine in the stratosphere. The correlation of Clegg and Brimblecombe (J. Phys. Chem. 1990, 94, 5369-5380; and 1994, 96, 6854) of the thermodynamic properties of aqueous HNO3 activities has been revised, and vapor pressure products (for the reaction HNO3. nH(2)O((cr)) reversible arrow HNO3(g) + nH(2)O((g)), where 1 less than or equal to n less than or equal to 3) assessed from literature studies. The activity product for the reaction HNO3. 2H(2)O((cr)) reversible arrow H-(aq)(+) + NO3 (-)((aq)) + 2H(2)O((1)) has also been determined. The model of Carslaw et al. (J. Phys. Chem. 1995, 99, 11557-11574) has been revised for the solubility of HBr in aqueous H2SO4 to stratospheric temperatures.