Active sites in titanium silicalite (TS-1) for selective oxidation of ethylene with H2O2 were studied by using ab initio quantum chemical methods. Models of the active sites were energy optimised and the interaction with H2O and H2O2 calculated. The binding situation of the resulting titanium hydroperoxo compounds were explained and transition states and activation energy for the epoxidation of ethylene were computed. The transition states were characterised by calculating the vibrational frequencies.