A systematic investigation of the exposure of nitric acid trihydrate (NAT) crystals to HCl is carried out using Fourier Transform Reflection Absorption Infrared spectroscopy (RAIR). The study deals with both phases of NAT, exposed to HCl at a range of temperatures between 87 and 165 K, and two pressure values, 10-4 and 10-5 mbar. For the α phase, HCl is mainly adsorbed at the surface at 87 K but dissociates and enters the lattice at higher temperatures. The ¿ phase behaves similarly at the lower temperatures but does not show any interaction with HCl at 165 K. A pressure of 10-4 mbar of HCl is necessary to induce any effects on NAT crystals. It is shown that traces of water greatly increase the HCl interactions.