One of the important objectives of the SUbsonic aircraft: Contrail and Cloud Effects Special Study (SUCCESS) field campaign data analysis is to investigate retrieving cloud microphysical properties using remote sensing observations. This paper presents the results of an infrared based retrieval of effective particle radius Re using brightness temperature observations near 8.5, 11 and 12 µm. The retrieval method relies on comparing 8.5-11 and 11-12 µm observed brightness temperature differences to theoretical simulations. During SUCCESS, conducted in April-May 1996 out of Salina KS, the MODIS Airborne Simulator (MAS) on the NASA ER-2 made observations of contrails, cirrus and mountain lee wave clouds. Observations indicate that contrail and cirrus clouds are distinguishable by their radiative properties. Retrieval of R-e for the lee wave cloud case on 2 May agrees with in situ observations from probes mounted on a NASA DC-8 aircraft, validating the infrared retrieval. The addition of the 8.5 µm information to the Re retrieval greatly enhances the sensitivity of the retrieval at small particle sizes (< 10 µm).