Methylations of bibenzonorbomenylidene episulfides 5 and 6 by Me3O+BF4-at -18 degreesC famished the corresponding episulfonium salts 7 and 8, respectively, with retention of the configuration of the episulfides, whereas those at room temperature or above produced a mixture of 7 and 8 in the ratio of about 1:4. Dynamic H-1 NMR investigation in the temperature range 25-40 degreesC allowed the determination of the activation parameters of the isomerization between 7 and 8, which obeyed first-order kinetics.