1.The reaction of 4,4-dichloro-3-buten-2-one with sodium phenoxide and with sodium phenyl sulfide was studied. The course of the reaction depends on the basic character of the medium and on the order in which the reactants are added.2.The replacement of one of the chlorine atoms of 4,4-dichloro-3-buten-2-one by the phenylthio or phenoxy group occurs preferentially in the cis position with respect to the acetyl group.