Polar stratospheric clouds were frequently observed by lidar at the Amundsen-Scott South Pole Station during May--October 1988. The dependence of the bakscattering cross section on the temperature can be referred to transitions of the HNO3/H2O system: it appears possible to distinguish the pure trihydrate from the mixed ice-trihydrate phase in the composition of the aerosol and, in some cases, to bracket the HNO3 and H2O content of the ambient gas, and to provide indications on the size of the particles. ¿ American Geophysical Union 1991