A computational study was performed on the interaction of sulfur trioxide with hydrogen halides. The MP2 level of theory was employed with LANL2DZ and Stuttgart effective core potential basis sets. Geometry optimizations, vibrational frequency calculations, and Natural Bond Orbital analyses were carried out on the SO₃-HX complexes, X= F, Cl, Br, and I. Trends in bond lengths, bond angles, and binding energies were analyzed. In the SO₃-HX complexes, the S-X bond lengths increased from HF to HI as the S-X-H bond angles decreased from HF to HI. The calculated binding energies of SO₃-HX were in the range of –2.0 to –6.2 kcal/mol with the SO₃-HF complex having the largest binding energy. The effect of catalytic water molecules on the conversion of the SO₃-HX complexes to halosulfonic acids, HSO₃X, also was investigated. With no water molecules present, the activation energies for conversion of SO₃-HX to HSO₃X ranged from 22.3 to 31.7 kcal/mol, with the SO₃-HI complex having the lowest activation energy. When one catalytic water molecule is added, a dramatic drop in activation energy of up to 17 kcal/mol was observed.