Nafion-H, a perfluorinated resin sulphonic acid was found to be an efficient and simple catalyst for the isomerization of epoxides to ketones or aldehydes depending on the nature of the substituents on the epoxide carbons. The reaction is very straightforward giving the products in high yield. Isoamylene oxide (5) gave isopropyl methyl ketone (3) and styrene oxide and its derivatives (7a-g) gave phenylacetaldehyde and its derivatives (8a-g).