The thiocarbonyl ylides 13 and 1,3-thiazol-5(4H)-thiones 1 undergo a smooth reaction to yield spirocyclic 1,3-dithioIanes 14-16 (Schemes 4-6) . The 1,3-dipolar cycloadditions occur in a regioselective manner, but the orientation of the thiobenzophenone-S-methylide (13b) differs from that of the cycloalkane thione-S-methylides 13a and 13c. Whereas the 1,3-cycloadduct with 13b is formed in accordance with frontier-orbital considerations, the inverse orientation in the reactions with 13a and 13c most likely is the result of steric hindrance in the transition state. The thiocarbonyl ylides have been prepared in situ from the corresponding 2,5-dihydro-1,3,4-thiadiazoles 12. The more stable aliphatic precursors 12a and 12c undergo decomposition at 50", the unstable 12b at -30".