The two Sigma and four Pi states of the;weakly bound complex Ar-NH3 correlating to j = 2, k = +/-1 ammonia have been observed by tunable far infrared difference frequency-microwave sideband spectroscopy. The results have been combined with published data to determine a new angular potential energy surface for the system. The barrier to threefold internal rotation of the NH3 about its C-3 axis in the complex is estimated to be 25.606(24) cm(-1) near the minimum energy (T-shaped) configuration. The potential also exhibits maxima at both symmetric top configurations, with energies approximately 53 and 31 cm(-1), respectively above that of the global minimum. The location and splitting between the symmetric and antisymmetric Sigma states are indicative of a strong interaction with another pair of unobserved states, most likely the first excited intermolecular stretch built on j = 1, k = +/-1 Ar-NH3.