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Ferroelectrics are electro-active materials that can store and switch their polarity (ferroelectricity), sense temperature changes (pyroelectricity), interchange electric and mechanical functions (piezoelectricity), and manipulate light (through optical nonlinearities and the electro-optic effect): all of these functions have practical applications. Topological switching of π-conjugation in organic molecules, such as the keto-enol transformation, has long been anticipated as a means of realizing these phenomena in molecular assemblies and crystals (1). Croconic acid, an ingredient of black dyes (2), was recently found to have a hydrogen-bonded polar structure in a crystalline state (3). Here we demonstrate that application of an electric field can coherently align the molecular polarities in crystalline croconic acid, as indicated by an increase of optical second harmonic generation, and produce a well-defined polarization hysteresis at room temperature. To make this simple pentagonal molecule ferroelectric, we switched the π-bond topology using synchronized proton transfer instead of rigid-body rotation. Of the organic ferroelectrics, this molecular crystal exhibits the highest spontaneous polarization (~20 µC [cm.sup.-2]) in spite of its small molecular size, which is in accord with first-principles electronic-structure calculations. Such high polarization, which persists up to 400 K, may find application in active capacitor and nonlinear optics elements in future organic electronics., A series of planar monocyclic [C.sub.n][O.sub.n.sup.2-] dianions called 'oxocarbons' have simple topologies that are highly symmetric ([D.sub.nh]) like benzene and the cyclopentadienyl anion (4). π-delocalization favours the dianion form' so [...]