3β, 19-Epoxy-3 α-methoxy-steroids ( e.g. 1, Scheme 1) are converted into the corresponding 3 α-methoxy-19-oxo compounds ( e.g. 3) in the presence of boron trifluoride etherate, via an intramolecular hydride ion transfer from C (19) to (3) which is shown to be an equilibrium process. By subjecting the specifically deuteriated (19 S)- and (19 R)-compounds 1b and 1c to this reaction it was shown to be highly stereoselective.