Tin-containing, medium pore molecular sieves (Si/Sn ratios above 40) with MEL structure have been synthesized hydrothermally and characterized by a variety of techniques. Changes in unit cell volume (XRD) account for about 20% of Sn4+ ions in possible tetrahedral positions. Sn-119 MASNMR spectra, however, indicate that these are mostly in octahedral coordination. Framework IR spectra show an absorption at 970 cm(-1) which is associated with Si-O-Sn vibration. Despite the uncertainty of the environment of the Sn4+ ions in the silicalite structure, these tin-silicates are found to be quite active in the hydroxylation of phenol and toluene with aqueous H2O2 as oxidant. The product distribution suggests that most of the Sn4+ ions are located within the channels of the MEL structure.