Infrared spectra of SO 3 and of its polymers in rare gas matrices were examined. While the in-plane vibrational modes showed little gas-matrix and matrix to matrix shifts, the out-of-plane ν2 vibration showed much larger medium sensitivity. This was attributed to formation of axial complexes with the more polarizable rare gases donating electrons to the SO 3 Lewis base. Several SO 3·X complexes were prepared and identified in doubly doped neon matrices. The observations suggest that the SO 3·OH 2 complex does not rearrange to H 2SO 4 in the 5 K neon matrix.