The retrieval and validation of the infrared measurements of the stratospheric aerosol layer derived from the improved stratospheric and mesospheric sounder (ISAMS) on board the Upper Atmosphere Research Satellite (UARS) are discussed in detail. The retrieval method is presented and an error analysis and sensitivity study are used to provide an error budget. The validation involves internal consistency checks, comparisons with coincident aerosol observations from other satellite-based instruments (cryogenic limb array etalon spectrometer (CLAES) and Stratospheric Aerosol and Gas Experiment II (SAGE II)) and in situ particle counters. The internal comparisons show that the precision of the 6.20-μm retrievals are better than 30% for the data on the 32-hPa and 22-hPa pressure surfaces and that the 12.11-μm retrievals are better than 20% and 25% for the data on pressure surfaces at 46 hPa and 32 hPa, respectively. Comparison of the ISAMS aerosol extinctions with the simultaneous CLAES measurements show systematic biases for both the 6-μm and the 12-μm retrievals with ISAMS generally measuring lower extinction values than CLAES. The comparison with calculated extinctions from balloon dustsonde data at 41 ¿N shows that the aerosol extinctions are systematically too high for pressures less than 40 hPa. ¿ American Geophysical Union 1996