Polar Ozone and Aerosol Measurement (POAM) III, a follow-on to the successful POAM II, is a spaceborne experiment designed to measure the vertical profiles of ozone, water vapor, nitrogen dioxide, and aerosol extinction in the polar stratosphere and upper troposphere with a vertical resolution of 1--2 km. Measurements are made by the solar occultation technique. POAM III, now in polar orbit aboard the SPOT 4 satellite, is providing data on north- and south-polar ozone phenomena, including the south-polar ozone hole, and on the spatial and temporal variability of stratospheric aerosols, polar stratospheric clouds, and polar mesospheric clouds. Differences between the POAM III and POAM II instruments are described. First validations of POAM III data products by comparison with Halogen Occultation Experiment and ozonesonde data are presented. ¿ 1999 American Geophysical Union