A partially purified protein obtained from a Flavobacterium sp. exchanges the halogen atoms of α-halo epoxides. The process has been shown to be reversible and its path delineated. The conversion of epibromohydrin into epichlorohydrin via the route: epibromohydrin + chloride ⇌ 1-bromo-3-chloropropanol ⇌ epichlorohydrin + bromide is illustrative. The conversion of halohydrins into epoxides by the enzyme is a stereospecific trans process. © 1969, American Chemical Society. All rights reserved.