Balloonborne aerosol soundings from McMurdo Station, Antarctica in 1990, documented the occurrence of polar stratospheric clouds (PSCs) during both fast and slow cooling events as well as the background stratospheric sulfate aerosol. PSCs formed under slow cooling predominated and in this case the size distributions were found to be bimodal with mode radii of 0.08 and 2--3 &mgr;m, similar to previous measurements in Antarctica. The aerosol soundings were also compared to lidar measurements at McMurdo in three cases. In the one PSC layer formed from fast cooling, the best agreement between measured and calculated scattering ratio was found using an index of refraction of 1.37, suggesting an amorphous nitric acid/water composition rather than crystalline nitric acid trihydrate. For the background aerosol, calculations using an index of 1.5 were generally in best agreement with the measured values.