Density functional theory calculations were performed on an activation of hydrogen peroxide over a cluster model of a titanosilicate catalyst. The calculation results showed possibility to form the hydrated peroxotitanosilicalite complex, containing a (Ti)-O-O-(Si) peroxo-moiety, as an oxidizing agent. Using this hydrated peroxo-titanosilicalite complex as an oxidizing agent, oxidation mechanisms were postulated for ethene epoxidation and for ammonia oxidation to form hydroxylamine. The ethene molecule was oxidized with the peroxo-oxygen coordinated to the central Ti atom of the hydrated peroxo-titanosilicalite complex, to form ethylene epoxide. For the ammonia oxidation process, ammonia replaced the adsorbed water molecule of the hydrated peroxo-titanosilicalite complex. The oxidation of the adsorbed ammonia in the (ammonia)-peroxotitanosilicalite complex led to the formation of an ammonia-N-oxide complex of the titanosilicalite catalyst model. The (ammonia-N-oxide) -titanosilicalite complex was transformed into the (hydroxylamine) -titanosilicalite complex, with a hydrogen transfer from the nitrogen to the oxygen of the ammonia-N-oxide moiety. The transition states were explored for these reaction processes. Using the peroxo-titanosilicalite complex containing a Ti-O-O-Si peroxo-moiety as an active oxidizing agent, the catalytic reaction mechanisms are proposed for ethene epoxidation and for ammonia oxidation to form hydroxylamine.