AbstractAlthough the chemistry of elusive tricyanomethane (cyanoform) has been studied during a period of more than 150 years, this compound has very rarely been utilized in the synthesis or modification of heterocycles. Three‐membered heterocycles, such as epoxides, thiirane, aziridines, or 2H‐azirines, are now treated with tricyanomethane, which is generated in situ by heating azidomethylidene‐malonodinitrile in tetrahydrofuran at 45 °C or by adding sulfuric acid to potassium tricyanomethanide. This leads to ring expansion with formation of 2‐(dicyanomethylidene)oxazolidine derivatives or creation of the corresponding thiazolidine, imidazolidine, or imidazoline compounds and opens up a new access to these push–pull‐substituted olefinic products. The regio‐ and stereochemistry of the ring‐enlargement processes are discussed, and the proposed reaction mechanisms were confirmed by using 15N‐labeled substrates. It turns out that different mechanisms are operating; however, tricyanomethanide is always acting as a nitrogen‐centered nucleophile, which is quite unusual if compared to other reactions of this species.