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The oxidation of benzaldehyde to benzoic acid with V2O5, SnO2‐promoted V2O5 and MnO2 as catalysts has been investigated. The reaction mechanism includes the following steps: dissociative chemisorption of benzaldehyde, yielding an asymmetric covalent benzoate complex with one oxygen atom of the sufrace, atomic rearrangement to the symmetric benzoate ion, desorption of benzoic acid and reoxidation of the catalyst surface by adsorption of oxygen. The activity of the oxides is found to be related to their ability to release oxygen. An antibatic relation between the Gibbs free energy of oxygen release and the catalytic activity is demonstrated. The action of the tin promoter in the mixed oxide catalyst is due to the loosening of the metal‐oxygen bond. A parallelism is suggested between the catalytic selectivity and the differential increase of the enthalpy of oxygen release with increasing degree of reduction.

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