The improved stratospheric and mesospheric sounder (ISAMS) used the pressure modulation technique to observe infrared emissions from the atmosphere at 7.4 μm and 7.8 μm. The target gases, methane and nitrous oxide, both emit at these wavelengths and so their concentrations have been determined by joint retrieval from the available signals. This paper describes the measurement technique, the current retrieval scheme (version 10), and the data set produced. The version 10 data set comprises typically 2600 profiles per day on 180 days between September 26, 1991, and July 29, 1992. Retrieved profiles extend in altitude from 7 mbar to 0.08 mbar for methane and from 7 mbar to 0.8 mbar for nitrous oxide. The precision of the data is better than 20% over much of the range and estimated systematic uncertainties are less than 30%. Comparisons with coincident measurements show that the systematic uncertainties are a reasonable estimate for the methane data but reveal a much larger positive bias for the nitrous oxide data relative to other measurements. Accounting for aerosol contamination effects and a priori biases, ISAMS methane data are recommended for use in scientific studies at altitudes between 7 mbar (5 mbar in the tropics) and 0.1 mbar. ISAMS nitrous oxide data are recommended for use between 7 mbar and 1.0 mbar where relative rather than absolute values are required. ¿ American Geophysical Union 1996