By unambiguous methods, (Z)- and (E)-2, 3-dimethyl(1, 1, 1, 4, 4, 4-²H₆)but-2-enes (3) were synthesized and transformed to the epoxides 4 with 3-chloroperbenzoic acids. Both the isotopomeric olefins and the epoxides are detected separately by ¹H-NMR at 400 MHz. Epoxidation of (Z)-3 with [RhICl(PPh₃)₃]/cumene hydroperoxide resulted in a 1:1 mixture of (Z)- and (E)-4, while reaction of (Z)-3 with [FeIII(tpp)]Cl/PhIO gave only (Z)-4 (tpp = tetraphenylporphyrin).