The ¿ phase of nitric acid trihydrate (¿-NAT) is assumed to be one of the essential components of polar stratospheric clouds (PSC) and may play an important role in ozone chemistry. Infrared spectra of vapour deposited films of ¿-NAT, have been measured in transmission and in reflection-absorption (RAIR). The spectra have been simulated with the different sets of optical constants available in the literature. The films are found to be optically isotropic. The refractive indices of Toon et al. (1994) give the best global fit to the data, but have low resolution and tend to smooth sharp features. The higher resolution indices of H¿pfner et al. (2006) lead to worse overall spectral simulations, but can account for the comparatively narrow feature at 820 cm-1 recently identified in field measurements of ¿-NAT particles. The best compromise is obtained with the "coa" indices of H¿pfner et al.