An analytic expression is given for the composition and volume of stratospheric HNO3--H2SO4--H2O aerosols in terms of the temperature and total amounts of H2O, HNO3 and H2SO4, taking into account conservation of HNO3. In contrast to previous parameterizations of equilibrium vapor pressures, the present scheme for calculating the aerosol composition does not require an iteration procedure, thus computation times are significantly reduced, making it ideal for inclusion in large-scale atmospheric models. Calculated compositions are compared with the results of a comprehensive thermodynamic model for all stratospheric conditions and maximum deviations are presented. ¿ American Geophysical Union 1995