The configurations of the spiro-hydantoins produced from 2-methyl- and 2-phenylcyclohexanone, 2,6-dimethylcyclohexanone, anddl-menthone by either the conventional Bucherer–Bergs (B•–B•) reaction, or a variant employing COS or CS2in place of CO2, have been established by13C nmr. In all cases the conventional B•–B•reaction gave the hydantoin having the 4′-carbonyl group equatorial, and the COS or CS2variant, the mono- or dithiohydantoin having the 4′-thiocarbonyl group, axial.