Oxidation of 2-thiabicyclo[2.2.1]heptane by a variety of oxidizing agents provided both endo and exo sulfoxides. The ratio of these sulfoxide isomers is a function of the relative stability of the sulfoxides and/or the mechanism of the oxidation reaction. It has been demonstrated that the exo oxide of 2-thiabicyclo[2.2.1]heptane is thermodynamically more stable than the corresponding endo oxide, a result not to be predicted from referenced researches in the field. © 1969.