1,3-Oxathiolane sulfoxide 4 in which the sulfoxide oxygen and the 2-methyl group are on the same face of the oxathiolane ring undergoes a sigmatropic rearrangement to produce a ring expansion product. The structure of this product would be dihydro-1,4-oxathiin 6 or isomeric exo compound 7. This paper describes physical and chemical methods to determine the correct structure of the two alternatives. Thus, $^1HNMR$, UV spectroscopies, and mass spectrometry showed that the product actually obtained had the structure 6. It was also found that from deuteration reactions of the product the compound 7 was initaly formed and then tautomerized to endo compound 6.