A new cyanogenic compound, sarmentosin epoxide, was isolated from the aerial parts of Sedum cepaea. Its structure was established mainly by 'H NMR and ' 3C NMR methods as 4-fi-D-glucopyranosyloxy-2,3-epoxy-2 hydroxymethylbutyronitrile. The new compound is not a typical cyanogenic glucoside but spontaneously releases HCN after hydrolysis of the oxiran group to a cyanohydrin in aqueous solution.