The interconversion between the two isomeric forms (cis and trans) of a series of 3-substituted thietane -1-oxide derivatives have been theoretically studied by means of ab initio molecular orbital theory. The optimized geometries were obtained at the HF/6-31G** level, whereas energies were calculated using several basis sets all including electron correlation at the second order Moller Plesset theory. To derive the thermodynamics of the cis <=> trans interconversion, the energies calculated at MP2/6-311++G** were used. To determine the effect of the substitution on C3 , the chloro, methyl, ethyl and t-buthyl derivatives have been studied. Ab initio calculations show that in all cases the cis isomer is more stable than the trans regardless the size of the substituent. The axial preference (trans) of the oxygen atom of the S=O group, found in six-membered heterocycles is not found in the four-membered thietane-1-oxide (TOX). The 1,3 repulsive interaction and the Hbeta - electron pair on sulfur repulsion are unimportant in the determination of the preferred geometry of these species. The Hbeta - O9 non-bonded interaction does determine the cis geometry to be the most stable one. &#9;A small solvent effect on the cis <=> trans interconversion is predicted for the 3-chloro and 3-methyl TOX, regardless the polarity of the medium. Temperature exerts an important role inthe 3-chloro-TOX system but just a small effect on the 3-alkyl derivatives. The use of the IR spectra in the S=O stretching modes regions is proposed to distinguish between cis and trans isomeric forms in 3-substituted TOX.