The ratio of isomeric epoxides obtained by alkaline hydrogen peroxide oxidation of (±)-carvenone and (±)-piperitone has been shown to be thermodynamically controlled. cis- and trans-Carvenone epoxides are of almost equal stability, but the preferential formation of the piperitone epoxide with the two alkyl groups trans(V) has been explained in terms of unfavourable steric interactions in the cis-form.