We report the spectrum of theν fundamental of chlorine dioxide centered in the infrared atmospheric window at 945.592 cm measured with essentially Doppler limited resolution at an instrumental line width of 0.001 cm using the Zürich prototype ZP2001 Bruker IFS 125 HR Fourier transform infrared spectrometer. The ro-vibrational line analysis is carried out with an improved effective Hamiltonian and a newly developed computer code ROVDES for the ro-vibrational spectra of open-shell free radical molecules including spin-rotation interactions. Accurate values of rotational, centrifugal and spin-rotational parameters were determined for O Cl O in the vibronic ground stateX B from more than 3500 ground state combination differences. The 7239 assigned transitions for theν fundamental withN = 76 andKmaxa 26 provide a set of 32 accurate effective Hamiltonian parameters for theν fundamental (v v v ) = (100) (21 rotational and centrifugal distortion parameters and 11 spin-rotational interaction parameters). This effective Hamiltonian (A- reduction andI - representation) reproduces 1703 upper state energies from the experiment with a root-mean-square deviationd = 1.67 × 10 cm and the 7239 transition wavenumbers withd = 3.45 × 10 cm . Our results provide a considerable improvement over previous results with which we compare and should provide a benchmark for theoretical studies with applications to atmospheric spectroscopy and laser chemistry, which are discussed in relation to our spectra. 1 1 1 1 1 2 3 rms rms −1 −1 16 35 16 2 max r −4 −1 −4 −1