Phase changes in stratospheric aerosols were studied by cooling a droplet of sulfuric acid (H2SO4) in the presence of nitric acid (HNO3) and water vapor. A sequence of solid phases was observed to form that followed Ostwald’s rule for phase nucleation. For stratospheric partial pressures at temperatures between 193 and 195 kelvin, a metastable ternary H2SO4-HNO3 hydrate, H2SO4.HNO3.5H(2)O, formed in coexistence with binary H2SO4.kH(2)O hydrates (k = 2, 3, and 4) and then transformed to nitric acid dihydrate, HNO3.2H(2)O, within a few hours. Metastable HNO3.2H(2)O always formed before stable nitric acid trihydrate, HNO3.3H(2)O, under stratospheric conditions and persisted for long periods. The formation of metastable phases provides a mechanism for differential particle growth and sedimentation of HNO3 from the polar winter stratosphere.