Both structural and solvent influences on the rate of oxidation of a series of aromatic aldehydes by chromic acid show the following features: the formation of a chromate ester is not a prerequisite to oxidation; in the rate-determining step of the reaction the C-H hydrogen is removed as a hydride anion. That the rate-dermining step involves a two-electron transfer has been proved by a study of the Induction Factor method. A tentative mechanism, based on an analogy to the well-known ‘S_N2’ process, has been proposed.