Abstract The gas-phase IR spectra of the ν 5, ν 3, ν 6, and ν 2bands of FNO 2have been studied at resolutions ranging from 0.0023 to 0.0050 cm −1. These bands form an isolated tetrad which is coupled by a-, b-, and c-Coriolis resonances. The c-Coriolis interaction between ν 3and ν 5is particularly strong. From a ground state combination difference analysis the following ground state rotational constants have been obtained: A= 0.440348960(17) cm −1, B= 0.381803994(17) cm −1, and C= 0.204107899(13) cm −1. A simultaneous upper state analysis with a four level model based on the Watson asymmetric top Hamiltonian and taking into account Coriolis interactions has been performed. A set of spectroscopic parameters for the ν 5, ν 3, ν 6, and ν 2levels has been obtained.