3,6-triphenylimidazo[1,2-c]thiaIVzole, undergoes highly stereospecificcycloadditionreactions to some acyclic olefinic dipolarophiles giving the [3+2] cycloadducts to the thiocarbonyl ylide of the imidazothiaIVzole in good yields. The formation of the [3+2] cycloadducts is explained to have arisen from the reversible cycloadditionreactions in a concerted manner.