The behavior of 5-phenyl-3H-1,2,4-dithiazole-3-thione (Ib) toward stable phosphorus ylides: methoxy-(IIIa), ethoxycarbonylmethylenetriphenylphosphorane (IIIb) and acetonylidenetriphenylphosphorane (IIIc) has been studied. The reactions take different paths leading to unusual products, depending only on the nature of the substituents of the ylides used. Structures of the new compounds were established on the basis of elemental analyses and spectroscopic studies. Some reaction mechanisms are proposed.