The effects of the addition of water to the reaction medium have been investigated for the epoxidation of cyclohexene by tert-butyl hydroperoxide catalysed by a series of TiO2-SiO2 mixed oxides prepared by a sol-gel procedure in which a fraction F of tetraethoxysilane is substituted by phenyltriethoxysilane. Infrared spectrometry gives a direct evidence of the retention of Si-C bonds by the solid. The substitution of OH groups by phenyls modifies the catalytic properties: for F=0, the solid is hydrophilic, inhibited by water and catalyses the hydrolysis of TBHP as competitive side reaction, whereas the solid obtained for F=0.2 is hydrophobic and not sensitive to a water/TBHP ratio as large as 1.