A rotational analysis of the 5ν1 (6450 A) band in the vibration–rotation spectrum of ammonia is presented here. The QP, QQ, and QR branches of a parallel transition are observed for values of J′ and K′ less than 6. Members of the SQ and OQ branches are observed with very low intensity. The molecular constants of the molecule in the (5, 0, 0, 0) vibrational level were determined; the N–H bond length is 1.015 ± 0.005 A and the bond angle is 112.0 ± 0.8° for this vibrational level. The potential energy to inversion of the molecule by excitation of the ν1 vibration only is 51 800 ± 1200 cm−1. The width of individual rotational lines increases with increase in J′ and K′. This increase in width is characteristic of a tunnelling mechanism. It may be due to the interaction between the normal modes of vibration at large vibrational energies. This analysis should improve the usefulness of the 6450 A band of ammonia for monitoring the temperature and pressure of the Jovian atmosphere.