The reactivity of isocyanic acid (HNCO) with solid ammonia (NH3) was first studied at 10 K, using FTIR spectroscopy. The ammonium isocyanate (NH4+OCN-) is formed from a reaction between HNCO and NH3. Vibrational band assignments for NH4+OCN- have been given. On the other hand, when HNCO is adsorbed on amorphous NH3 film, the reaction does not occur. Warming up of this sample at 90 K induces the NH4+OCN- formation. Quantum calculations showed that the solvation of NH3 directly bonded to HNCO by at least three NH3 molecules plays a major role in the NH4+OCN- formation process and confirmed the spontaneous character of this reaction. (C) 2002 Elsevier Science B.V. All rights reserved.