The uptake of HNO3 onto ice, HNO3-H2O layers, and solid H2SO4/H2O solutions were studied in a cylindrical flow tube in conjunction with a chemical ionization mass spectrometer detection system. An efficient loss of HNO3 from the gas-phase to these surfaces was observed and the uptake coefficient was ≥0.3. The HNO3-H2O layers formed, however, had HNO3 vapor pressures significantly in excess of the expected values for NAT (nitric acid trihydrate). Supersaturations in HNO3 of at least a factor of three and up to ten were observed suggesting a hindrance to the formation of NAT. These results imply that the condensation temperatures of Type I PSCs are 2--3 K colder than that expected for NAT. ¿ American Geophysical Union 1992