1.The electron impact method has been used to measure the relative ionization energy and appearance energy of [M-Me]+ions of all of the stereoisomers of 2-methyl-1-thiadecalin. It has been shown that the IE of the trans-linked epimers is lower than for the cis-linked epimers, and the IE and AE of [M-Me]+of the trans-thiadecalin with the axial 2-methyl group are respectively 0.04 and 0.07 eV lower than for the equatorial epimer.2.On the basis of the values of the AE of [M-Me]+, an estimate has been made of the difference in the heats of formation of the trans-linked 2-methyl-1-thiadecalins.