The m AE1 3 perpendicular band of 14 NF 3 (m m 0 ¼ 907:541 cm À1 ) has been studied with a resolution of 2.5 Â 10 À3 cm À1 , and 3682 infrared (IR) transitions (J max ¼ 55, K max ¼ 45) have been assigned. These transitions were complemented by 183 millimeterwave (MMW) rotational lines (J max ¼ 25, K max ¼ 19) in the 150-550 GHz region (precision 50-100 kHz). The kl ¼ þ1 level reveals a strong A 1 =A 2 splitting due to the lð2; 2Þ rotational interaction (q ¼ À4:05 Â 10 À3 cm À1 ) while the kl ¼ À2 and +4 levels exhibit small A 1 =A 2 splittings due to lð2; À4Þ and lð0; 6Þ rotational interactions. All these splittings were observed by both experimental methods. Assuming the v 3 ¼ 1 vibrational state as isolated, a Hamiltonian model of interactions in the D reduction, with lð2; À1Þ rotational interaction (r ¼ À1:96 Â 10 À4 cm À1 ) added, accounted for the observations. A set of 26 molecular constants reproduced the IR observations with r IR ¼ 0.175 Â 10 À3 cm À1 and the MMW data with r MMW ¼ 134 kHz. The Q reduction was also performed and found of comparable quality while the QD reduction behaved poorly. This may be explained by a predicted Coriolis interaction between v 3 ¼ 1 and v 1 ¼ 1 (A 1 , 1032.001 cm À1 ) which induces a slow convergence of the Hamiltonian in the QD reduction but has no major influence on the other reductions. The experimental equilibrium structure could be calculated as: r e (N-F) ¼ 1.