The liquid-solid phase transitions of emulsified binary HNO3/H2O and ternary HNO3/H2SO4/H2O acid solutions have been investigated using differential scanning calorimetry in order to elucidate the mechanism of ice formation from stratospheric aerosols. The results indicate that binary solution droplets with less than 33 wt % HNO3 freeze to form ice at temperatures 37-65 K below the equilibrium ice melting temperatures. Droplets with higher concentrations (up to 65 wt %) do not freeze; instead, they form glasses at temperatures below 160 K. The results also show that the presence of H2SO4 in amounts corresponding to less than 5 wt % depresses the freezing points of the 0-33 wt % HNO3 droplets by 5 to >30 K, in agreement with earlier observations. The implications of the new data for our understanding of the formation of polar stratospheric clouds have been investigated using a thermodynamic model. The results indicate that an ice saturation ratio of greater than 1.65 is required for ice particles to form from ternary aerosols at stratospheric temperatures, corresponding to a cooling of about 3 K below the ice frost point.