In this study, we use thermal infrared channels to retrieve the optical thickness and effective particle radius of ice clouds. A physical model is used in conjunction with Atmospheric Infrared Sounder (AIRS) temperature and water vapor profiles to simulate the top-of-atmosphere (TOA) brightness temperatures (BTs) observed by the Moderate Resolution Imaging Spectroradiometer (MODIS) for channels located at 8.5, 11.0, and 12.0 µm (1176, 909, and 833 cm-1). The model is initially validated by comparing simulated clear-sky BTs to MODIS-observed clear-sky BTs. We also investigate the effect of introducing a +3 K bias in the temperature profile, a +3 K bias in the surface temperature, and a +20% bias in the water vapor profile in order to test the sensitivity of the model to these inputs. For clear-sky cases, the simulated TOA BTs agree with MODIS to within 2-3 K. The model is then extended to simulate thermal infrared BTs for cloudy skies, and we infer the optical thickness and effective radius of ice clouds by matching MODIS-observed BTs to calculations. The optical thickness retrieval is reasonably consistent with the MODIS Collection 5 operational retrieval for optically thin clouds but tends to retrieve smaller particle sizes than MODIS.