A mechanism for the conversion of stratospheric N2O5 to HNO3 by the reaction of N2O5 with aqueous aerosol in the winter polar stratosphere is proposed to explain the observed deficit amounts of dinitrogen pentoxide and excess amounts of nitric acid compared to the results of calculations based on current gas phase chemistry. The presence of large numbers of aqueous aerosols in the winter polar vortex is associated with the formation of polar stratospheric clouds. An estimate of the rate of the conversion reaction leads to a time constant of about 18 days; model calculations indicate that a conversion process with this time constant is sufficient to convert about 4 ppbv of NO2 into NHO3 in the polar winter vortex.