The freezing of a submicron-sized aerosol composed of H2O and HNO3 in a precise 2:1 concentration ratio has been measured using Fourier transform infrared extinction spectroscopy. The measurements were carried out in a flow tube operating at temperatures and pressures appropriate to the polar stratosphere. On the timescale of this measurement, about 15 s, detectable nucleation occurred at 179¿1.6 K. Ten percent of the sample was frozen after 15 s at a temperature of 178.8 K; 50% was frozen at 177.5 K, and 90% was frozen at 175.8 K. Using the known (constant) aerosol flow velocity, the nucleation rate constant was obtained from the freezing point measurements. Values of this rate constant are reported over the temperature range between 176 K and 179 K. In this range the freezing temperature is in excellent agreement with that measured by Barton et al. <1993>, and the temperature dependence of the nucleation rate constant agrees well with that calculated using the method of MacKenzie et al. <1997>. It does not agree with that reported by Tisdale et al. <1997>. Extrapolation of the rates indicate that nitric acid dihydrate nucleation from liquid aerosol droplets having a 2:1 H2O:HNO3 composition would occur on the stratospherically relevant timescales of 1 hour and 1 day at temperatures of 183 and 185 K, respectively. ¿ 1998 American Geophysical Union