The interaction between adsorbed NH 3 and adsorbed CO molecules on two Ni single crystal planes has been investigated using ESDIAD and temperature programmed desorption (TPD). Interactions have been observed on both surfaces which influence the ESDIAD patterns of both adsorbed species. Evidence for long distance azimuthal orientation interactions of NH 3 with CO on Ni(110) is observed, whereas shorter distance interactions are observed on Ni(111). In the case of the short distance CO…NH 3 interactions on Ni(111), a tipping of the C 3v axis of NH 3 away from the normal is seen. The role of the substrate crystal structure is shown to be important in determining the character of the intermolecular interactions on the two surfaces.