We calculate the formation rate of sulfuric acid-water aerosol particles as a function of altitude for the conditions of the winter Arctic and Antarctic stratospheres. The theoretical results indicate that sulfate particle formation can occur in the polar winter stratosphere. Conditions for new particle formation are increasingly favorable as the altitude increases between 20 and 30 km because of the decrease in surface area of pre-existing particles and increasing sulfuric acid vapor supply. The theoretical predictions are consistent with observations of a high altitude CN layer over Antarctica in the spring. Available vapor pressure data indicate that ternary system particles composed of sulfuric acid, nitric acid and water are not thermodynamically stable under winter stratospheric conditions. ¿ American Geophysical Union 1990