Eight new intermolecular vibrational states of Ar-NH3 have been observed by tunable far infrared laser vibration-rotation-tunneling spectroscopy (FIRVRTS) between 28 and 45 cm(-1). These states provide a detailed measure of the angular-radial coupling in the intermolecular potential energy surface (IPS) and permit the dependence of the IPS on rotation about the NH3 symmetry axis to be quantitatively assessed for the first time. The determination of a complete three-dimensional IPS for the complex is described in the accompanying paper.