The 5(05)<--4(04), 4(04)<--3(03), 3(03)<--2(02) and 2(02)<--1(01) rota
tional transitions of the open-shell van derWaals complex ArNO2 have b
een observed using a Fourier-transform microwave spectrometer. The com
plex was produced by supersonic expansion of NO2 gas in argon. The spe
ctrum showed fine, magnetic hyperfine and electric quadrupole structur
e. The data have been fitted to a semirigid Hamiltonian to determine t
he molecular parameters for the K(A)=0 state. We discuss the implicati
ons of these derived parameters for the electronic and geometric struc
ture of the complex.