Titanium-containing zeolites are a class of porous materials having widespread use in industry as catalysts for redox processes in mild conditions. At low titanium concentrations, titanium isomorphously replaces silicon in the tetrahedral sites of the zeolitic framework. Titanium may expand its coordination from four to six, acting as a Lewis acid with respect to bases adsorbed in the channel-and-cage systems typical of zeolites. Herein, we interpret the red-shift (bathochromic effect) of the ligand-to-metal-charge-transfer (LMCT) electronic transitions detected by UV/Vis studies on titanium zeolites upon hydration at the microscopic level by using a DFT-based computational approach which takes into account the full periodicity of the crystalline phase. The relationships among the structures of the zeolitic titanium sites, the adsorbed water molecules and the electronic excitation properties in models of titanium zeolites are discussed. The LMCT bands' profiles, edges and intensities are interlaced with both the location and the coordination of the water molecules at the titanium site.Since the first patent in 1985, [1] titanium silicalite (TS-1) has been widely adopted in partial oxidations of hydrocarbons by using aqueous hydrogen peroxide as oxygen source.[2] In asprepared TS-1, titanium is pentacoordinated, via four framework oxygen atoms and one OH À group.[3] The OH À group is released upon calcination, and, in evacuated (solvent free) zeolites, titanium is coordinated by the four framework oxygen atoms in a tetrahedral geometry. UV/Vis and EXAFS experiments show that water loading causes a reversible expansion of the coordination shell of titanium. Diffuse reflectance UV/Vis spectroscopy indicates a red-shift of a multiple band, in thẽ 200-240 nm range in dry samples, [4] and from~200-250 nm in hydrated TS-1.[5] EXAFS experiments confirm that such a redshift occurs along with the formation of penta-and/or hexacoordinated titanium centres. [5,6] Absorption in these ranges, which are characteristic of titanium sites in zeolites, are due to a LMCT mechanism. [7] While formation and reactivity of oxidizing intermediates in TS-1 have been the subject of many studies, the relationships between the extent of hydration, structure and electronic excitation spectra have received less attention. A description of the electronic excitation in titanium zeolites in non-oxidizing conditions at an atomistic level is, however, a topic of current interest, as it has been recently reported that titanium zeolites might replace the semiconducting TiO 2 layer in photovoltaic cells. [8] Most of the reported data deals with experiments on TS-1, a zeolite with an MFI framework structure [9] and a unit cell content of [Ti x Si 96Àx O 192 ]-too large for extensive ab initio calculations. As a consequence, a number of theoretical studies have been performed on model clusters or by using embedded cluster approaches (see for example, ref.[10]). Smaller titanium zeolites have been studied by ab initio methods by taking into account perio...