Some of the mechanistic studies on dehydrogenation, dehydration and decarboxylation over metal oxide catalysts are reviewed. The type of activity of a catalyst is determined by the nature of the surface-substrate interaction, the same catalyst giving rise to different relative activities depending on the substrate. In addition to dehydrogenation and dehydration reactions many oxide catalysts promote hydrogen transfer reactions too. In the mixed oxide systems the interfaces between the pure oxides as well as interfaces arising from new phases formed by the interaction of component oxides are active regions. In the ketonization of acids the same active sites as those effective for the decomposition of alcohols are involved. The type of the intermediate depends on the conditions of reaction, the most important being the temperature. The technique of competitive reactions has been used to establish that the catalytic ketonization is a bimolecular process on the surface of the catalyst. © 1984 Indian Academy of Sciences.