High-resolution (0.001 cm -1 ) coherent anti-Stokes Raman scattering (CARS) was used to observe the Q-branch structure of the IR-inactive Ξ½ 1 symmetric stretching mode of 32 S 16 O 3 and its various 18 O isotopomers. The Ξ½ 1 spectrum of 32 S 16 O 3 reveals two intense Q-branches in the region 1065-1067 cm -1 , with surprisingly complex vibrational-rotational structure not resolved in earlier studies. Efforts to simulate this with a simple Fermi-resonance model involving Ξ½ 1 and 2Ξ½ 4 states do not reproduce the spectral detail, nor do they yield reasonable spectroscopic parameters. A more subtle combination of Fermi resonance and indirect Coriolis interactions with nearby states, 2Ξ½ 4 (1 = 0, Β±2), Ξ½ 2 + Ξ½ 4 (1 = Β±1), 2Ξ½ 2 (1 = 0), is suspected and a determination of the location of these coupled states by high-resolution infrared measurements is under way. At medium resolution (0.125 cm -1 ), the infrared spectra reveal Q-branch features from which approximate band origins are estimated for the Ξ½ 2 , Ξ½ 3 , and Ξ½ 4 fundamental modes of 32 S 18 O 3 , 32 S 18 O 2 16 O, and 32 S 18 O 16 O 2 . These and literature data for 32 S 16 O 3 are used to calculate force constants for SO 3 and a comparison is made with similar values for SO 2 and SO. The frequencies and force constants are in excellent agreement with those obtained by Martin in a recent ab initio calculation.