Microwave and infrared spectra of the open-shell complex ArNO2 have been recorded. The microwave spectrum (6-18 GHz) consisted solely of the a-type transitions 5(05)<--4(04), 4(04)<--3(03), 3(03)<--2(02), and 2(02)<--1(01) involving the K-a=0 state and 5(23)<--4(22), 5(24)<--4(23), 4(22)<--3(21), 4(23)<--3(22), 3(21)<--2(20), 3(22)<--2(21), involving the K-a similar or equal to 2 state. These transitions showed structure due to fine, magnetic hyperfine and electric quadrupole interactions. The infrared spectrum, associated with the nu(3) asymmetric vibrational mode of the NO2 monomer, consisted of three bands (P-R(0), (R)Q(0), and (R)R(0) and both K doublets of P-P(2), (P)Q(2) and (P)R(2)) centered around 1615 cm(-1). The data have been fitted to a semirigid Hamiltonian to determine the molecular parameters. The derived parameters are analyzed in terms of those of the free NO2 radical. Changes in these parameters upon complexation can be caused by a geometric effect due to the rotation of the inertid axes from the monomer to the complex, and an electronic effect caused by a distortion of the electronic wave functions on complex formation. The electronic changes (which may give an indication of incipient chemical bond formation) are shown to be very small. The absence of odd K-a " states in both the infrared and microwave spectra was rationalized in terms of a high frequency tunneling motion of the NO2 within the complex. Both a dynamics calculation and a model potential based on atom-atom interactions provided additional support for a nonplanar equilibrium structure with a low barrier to planarity.