The freezing behavior of aqueous nitric acid solutions was investigated in order to elucidate the formation mechanism of solid polar stratospheric clouds (PSCs). Drops with composition ranging from 40 to 60 wt % HNO3 were prepared and their phase transitions were monitored with an optical microscope. Homogeneous nucleation rates of nitric acid dihydrate (J(NAD)) and nitric acid trihydrate (J(NAT)) at temperatures between 175 and 195 K were estimated from the data. Classical nucleation theory was used to parametrize the results into simple equations to calculate J(NAT) and J(NAD) for different temperatures and concentrations of the liquid. The nucleation rate of the nitric acid hydrates was found to depend predominantly on the saturation ratio of the liquid with respect to the solid: higher saturation ratios correspond to higher nucleation rates. Both NAD and NAT can preferentially nucleate in binary nitric acid solutions, depending on the temperature and the composition of the liquid; also, NAD appears to catalyze the nucleation of NAT below similar to 183 K. The results suggest that the largest drops in a PSC will freeze homogeneously if the stratospheric temperature remains near 190 K for more than 1 day, forming mixed liquid-solid clouds. In addition, the results indicate that nonequilibrium quasi-binary nitric acid solutions will not freeze in the stratosphere unless the temperature drops below 180 K.