Fourier Transform Infra-Red (FTIR) analysis of thin ice films was performed in order to investigate the influence of Nitric Acid Trihydrate (NAT) coating on the lifetime of stratospheric ice particles in air subsaturated with respect to ice. Two sorts of experiments were performed in which NAT was either condensed from the gas phase onto preexisting ice films or was incorporated into growing ice films, which subsequently were subject to slow evaporation under controlled vapor pressure conditions. Neither a thermodynamically efficient sealing of the ice nor a significant kinetic slowing of its evaporation was observed. A possible alternative mechanism leading to a more efficient NAT coating is proposed. ¿ 1998 American Geophysical Union