The configuration of the stereoisomeric l-oxa-4thia-8-t-butylspiro [4.5] decanes (4- tbutylcyclohexanone ethylene monothioketals, I and 11) previously assigned on the basis of nmr data has been confirmed through oxidation rate data of the corresponding sulfoxides with perbenzoic acid. The presumed equatorial sulfoxide reacts about twice as fast as the axial. Repetition of the lithium aluminum hydride reduction of the monothioketals gave, ...