Epoxides are converted selectively to the corresponding carbonyl-containing compounds by reacting the epoxide with an oxidizing agent, typically hydrogen peroxide, in a liquid aqueous/organic two-phase system comprising:(a) an organic phase substantially containing the epoxide, and(b) an aqueous acidic phase substantially containing the oxidizing agent, in the presence of an onium compound capable of achieving phase partitioning and a catalytic system comprising a first catalyst component which is at least one element selected from tungsten, molybdenum, vanadium and chromium, or a compound containing at least one of the aforesaid elements, and a second catalyst component which is a phosphorus (V) acid or a species convertible to a phosphorus (V) acid.