This work investigates thermal emission from polar stratospheric clouds (PSCs) measured at 10.9 and 11.9 μm wavelengths (channels 4 and 5) by the advanced very high resolution radiometer (AVHRR). PSCs can be broadly categorized by particle composition as either ice or nitric acid mixtures. This work shows that nitric acid PSCs are invisible to AVHRR, while some ice PSCs can be detected. Methods were developed to discriminate ice PSCs from other cloud types in AVHRR imagery based on the brightness temperature difference between channels 4 and 5. When PSCs are identified in AVHRR imagery, it is possible to estimate the PSC optical depth, effective radius, and ice water path using relationships reported here, however, these estimates may have large uncertainties. ¿ 2001 American Geophysical Union