Microwave and radio frequency spectra of ArSO3 and radio frequency spectra of N2SO3 are measured by molecular beam electric resonance spectroscopy. Both complexes are symmetric rotors. The spectroscopic constants determined for Ar32SO3 are B = 1449.089(5) MHz, DJ = 3.6(6) kHz, and μ = 0.2676(3) D. For Ar 34SO3, B = 1439.090(5) MHz. The stretching frequency of the weak Ar-S bond is 61.(5) cm-1, and the force constant is thus 0.059(10) mdyn/Å. The Ar-S bond length in ArSO3 is 3.350(1) Å. The Ar-S-O angle is found to be 90.5°(5), thereby indicating no significant out-of-plane distortion of the SO3 moiety. Preliminary assignment of the N2SO3 spectrum yields a dipole moment of 0.46(1) D. Electric deflection studies show that (SO3)2 and (SO3)3 are polar species, while Xe2SO 3 is not.