Recent observations of type I polar stratospheric cloud particles (PSC I) reveal some discrepancies with data on nitric acid trihydrate (NAT) obtained in laboratory studies. To better understand the in situ growth of PSC I particles, this experiment condensed mixtures of water and nitric acid vapor at temperatures and pressures which approximated those of the polar stratosphere more closely than in prior studies. Mono-, di-, and trihydrates of nitric acid were observed in solids condensed from vapor mixtures relatively rich in HNO3. Solids formed under conditions that approached stratospheric contained significantly less HNO3 than the 25 mol% expected of NAT; the composition appeared to slowly change toward that of NAT over about ten hours. Solids with low HNO3 content showed HNO3 vapor pressures that were often highly supersaturated with respect to NAT. These results may help explain recent arctic PSC observations as those of relatively young clouds whose condensed materials have not yet reached equilibrium. ¿ American Geophysical Union 1993