Density Functional Theory (DFT) calculations at the B3LYP/6-31+G* level have been performed on 5-thiabicyclo[2.1.1]hex-2-ene S-oxide derivatives. The geometrical and electronic properties of the compounds have been analysed in order to explain the favoured stability of the exo configuration. Isodesmic reactions at the Gaussian-G2 theory yielded the exo conformer as the most stable one. Moreover, the NMR chemical shift parameters (GIAO method) together with the Atoms in Molecules theory reveal an stabilisation of the S atom with the double bond for the exo configuration, in agreement with experimental results.