Chlorine nitrate has two low-lying vibrational modes that lead to a series of Fermi resonances in the 9υ97υ7 family of levels that include the 92⇔71 and 93⇔7191 dyads and the 94⇔9271⇔72 and 95⇔9371⇔9172 triads. These states, along with the ground and 91 vibrational states, have been previously analyzed with millimeter and submillimeter wave spectroscopy and provide a substantial body of data for the investigation of these resonances and their impact on calculated spectroscopic constants and structural parameters. Due to fitting indeterminacies, these previous analyses did not include the main Fermi resonance interaction term. Consequently, the fitted rotational constants are linear combinations of the unmixed rotational constants of the basis vibrational states. In this paper, we have calculated the contributions of the Fermi resonances to the observed rotational constants in a model that determines the vibrational–rotational constants, the Fermi term and the mixing between interacting vibrational states, the cubic potential constant (φ997) that connects interacting levels through a Fermi resonance, and the inertial defects. These results agree with predictions from ab initio and harmonic force field calculations and provide further experimental information for the determination of the fundamental molecular properties of chlorine nitrate.