[1] The relative extinction coefficients for Arctic polar stratospheric clouds (PSCs) are determined using a transmittance-ratio technique from spectral data observed in early 1997 by the Improved Limb Atmospheric Spectrometer (ILAS). The observed relative extinction coefficients are compared with theoretical coefficients calculated by the Mie theory for single mode lognormal size distributions of PSCs. The observed PSCs are then classified into three types: nitric acid droplets (HNO 3 /H 2 O), and nitric acid trihydrate (NAT) solid particles in the form of a-NAT and b-NAT. However, the definitive detection of a-NAT is difficult due to uncertainties of atmospheric temperature measurements and laboratory determination of refractive indices.