An unanticipated exo-cyclic nucleophilic attack of thiol reagents on vinyl cyclic carbonates is described under DBU catalysis. The process allows the production of allylic thioethers under moderately mild conditions following a sequence of steps that involve a nucleophilic attack of the thiol on the C-terminus of the CC bond, carbonate ring-opening, and decarboxylation followed by final protonation. This unusual thiol-induced ring-opening complements the previously reported and more widely observed endo-cyclic nucleophilic attack on either the carbonate or methylene carbon centers, and therefore amplifies the reactivity modes that can be accessed with these heterocycles.