The catalytic behaviour in the ammoximation reaction of cyclohexanone to cyclohexanone oxime and the presence of activated forms of molecular oxygen on different commercial silicas (Akzo F-7, Degussa A200, Cabosil) was investigated. The presence of O-2(-) species in the same range of temperature at which the silicas are active in ammoximation reaction was detected. The investigated silicas presented different selectivities in cyclohexanone oxime but all of them gave a high amount of tars as by products in presence of molecular oxygen. It has been proposed that the O-2(-) adsorbed on the surface of silica are only involved in the beginning a radical reaction responsible of the formation of tars and that the high activity in oxime formation presented by Akzo F-7 is mainly related to its mesoporosity and the presence of tars.