We present Microwave Limb Sounder (MLS) measurements of gas-phase HNO3 obtained at the beginning of five southern hemisphere winters: 1992--1996. To investigate the composition of the polar stratospheric clouds (PSCs) that formed in early winter each year, the observed evolution of HNO3 at 465 K is compared against that predicted using nitric acid trihydrate (NAT), nitric acid dihydrate (NAD), and liquid ternary solution models of PSC formation and correlated with temperature histories from three-dimensional back trajectory calculations. The MLS HNO3 observations suggest that the initial composition of PSCs depends on the physical state of the background sulfate aerosols. If the preexisting aerosols are liquid, then the formation of ternary solutions is initiated as the temperature drops below about 192 K, followed by a gradual conversion to NAD after exposure to low temperatures for several days. HNO3 uptake into ternary solutions occurs at higher temperatures, and the conversion to NAD is delayed, under conditions of enhanced aerosol loading from the Mount Pinatubo eruption. If a majority of the background aerosols are frozen, the growth of ternary solutions is inhibited, but formation of a metastable, water-rich, HNO3-containing solid phase characterized by a relatively high HNO3 vapor pressure (type Ic PSC) may occur. In general, MLS HNO3 measurements obtained during early southern winter indicate a strong correspondence between the area of gas-phase HNO3 loss and the area of temperatures below 192 K but only a weak correspondence between the area of gas-phase HNO3 loss and the area of temperatures below 195 K, the value commonly assumed as the threshold for PSC formation. Although temperatures were low enough to maintain NAT PSCs, the MLS data show that they were not forming, at least not over spatial scales comparable to or larger than the ~400¿100¿5 km MLS field of view. ¿ 1998 American Geophysical Union