Upon loading of aqueous hydrogen peroxide into Ti silicalite sieve and removal of the solvent by evacuation, a thermally stable hydroperoxo species was detected by infrared spectroscopy with an OO stretch absorption at 837 cm(-1) (110, 793 cm(-1)) and a broad OH band at 3400 cm(-1). The species, assigned to TiOOH (eta(2)), is photolabile and is very efficiently converted to a titanol group upon excitation of its UV-vis ligand-to-metal charge-transfer (LMCT) absorption. Growth of H2O (160, 1629 cm(-1); 1110, 1625 cm(-1)) and of the 960 cm(-1) framework mode indicate that a substantial traction of the titanol groups recondense with adjacent Si-OH to Ti-O-Si. The TiOOH moiety was shown by in situ FT-IR monitoring to oxidize small olefins at room temperature in the dark (propylene) or under photoexcitation of the LIMCT chromophore (ethylene). This is the first direct detection of the active oxidation site in H2O2-loaded Ti silicalite sieve.