Polar Ozone and Aerosol Measurement (POAM) III,a follow-on to the successful POAM II, is a spaceborne experiment designedto measure the vertical profiles of ozone, water vapor, nitrogen dioxide,and aerosol extinction in the polar stratosphere and upper troposphere witha vertical resolution of 1-2 km. Measurements are made by the solaroccultation 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 stratosphericaerosols, polar stratospheric clouds, and polar mesospheric clouds.Differences between POAM III and POAM II instruments are described.First validation of POAM III products by comparison with HalogenOccultation Experiment and ozonesonde data are presented.