The cis double left right arrow trans isomerization reaction of dithietane-1,3-dioxide (DTDO) has been studied by means of theoretical methods in the framework of molecular orbital theory. The energetics of this reaction was obtained with several basis sets and levels including both polarization and diffuse functions. Electronic correlation has been taken into account by applying second-order perturbation Moller-Plesset theory. The thermodynamics indicates that both isomeric forms are in almost the same concentration in the gas phase, whereas in solution in more polar solvents, the cis form becomes more important. The transition state for the cis double left right arrow trans interconversion reaction has been fully characterized and the activation energy has been calculated to be 47.8 kcal/mol in the gas phase. In general, the solvent, simulated by the PCM model, lowers E a as its polarity increases, and in DMSO (epsilon = 46.7) Ea goes down by ca. 20 kcal/mol. The calculated H-1-NMR chemical shifts are consistent with the experimental data. The predicted O-17 and S-33-NMR spectra should allow to distinguish between the two conformations.