An understanding of the interaction of ammonia (NH 3 ) with dust grain surfaces is necessary in order to model the chemistry of the interstellar medium. With this in mind, a detailed reflection absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD) study of the adsorption of NH 3 on highly oriented pyrolytic graphite (HOPG) has been performed at ∼90K. Very sharp bands are observed in the RAIR spectrum, which are characteristic of the formation of crystalline NH 3 on the HOPG surface. TPD spectra show that, unusually, the monolayer of NH 3 desorbs at a lower temperature than the multilayer. This can be attributed to the extensive hydrogen bonding that occurs in the multilayer. Kinetic parameters, including the desorption order, desorption energy and pre-exponential factor, have been determined for this adsorption system for the first time.