Spectral observations of cirrus clouds can be analyzed to obtain cloud particle effective radii using measurements at select wavenumbers in the infrared atmospheric window. The suitability of these wavenumbers to determine a range of sizes is constrained by the dependence on particle radius of ice absorption efficiency (Qabs). By using only those wavenumbers at which Qabs varies monotonically with radius, cloud particle radii up to approximately 20 ¿m can be determined; larger values, however, present problems. At some wavenumbers, the variation in Qabs is not sufficiently large to distinguish between large particle radii. At other wavenumbers, larger particles are more easily distinguished, but Qabs values do not correspond to unique particle radii. Using one set of wavenumbers only to determine whether the particle is large or small, and a second set to actually determine the effective radius of a large cloud particle, the limitations faced at individual wavenumbers can be partially overcome. This is demonstrated using yearlong spectral observations of ice clouds at South Pole station.