An oxidation system consisting of methyltrioxorhenium and H2O2 catalyzes the cleavage of CC-double bonds. By varying the reaction conditions, it is possible to generate either the aldehyde or the carboxylic acid in reasonable-to-excellent yields and selectivities, depending on the starting material. The application of these findings to isoeugenol and trans-ferulic acid - both available from renewable resources - results in a highly efficient synthesis of vanillin.