The ammoximation in gas phase of cyclohexanone is a reaction with an oxidation step performed on an insulator oxide such as amorphous silica. Conventional mechanisms for such a kind of reaction are not useful. This paper reviews the efforts carried out to understand the catalytic behavior of the silica and highlights the particular catalytic features of the tars, the carbonaceous products formed during the ammoximation, in the oxidation step of the reaction. The catalytic behavior and the role of the acid sites, of the tars and of the activated forms of molecular oxygen were investigated on several commercial amorphous silicas and on pure and doped silicas prepared by sol-gel method. The silica catalyst shows a bifunctional behavior : acid sites are necessary for the formation of imine, the intermediate of the reaction, and oxidizing sites for the formation of oxime. For the first step the presence of silanols of the right acidity results is essential. For the oxidative step the carbonaceous products formed during the reaction seem to play an essential role. It has been suggested that the tars are formed by a polymerization reaction which occurs at the silica surface and involves, in the initiation stage, activated oxygen species generated by the silica surface. The reaction progression with time involves increasing amounts of the carbonaceous products, with a corresponding decline in access to the active sites by pore blockage and, as a consequence, with a corresponding decline in the catalytic activity.