FTIR spectroscopy was used to study the influence of pre-treatment on OH groups of H-ZSM-5, Ga-ZSM-5, Pt-ZSM-5 and GaPt-ZSM-5 catalysts and the confinement of benzene molecules in these catalysts pre-treated under different conditions. Types of the hydroxyl groups in GaPt-ZSM-5 and Ga-ZSM-5 are very sensitive to the pre-treatment of the samples. Besides the IR bands at 3745, 3665 and 3612 cm(-1) assigned to terminal silanol Si-OH, OH group attached to extra-framework aluminum and bridged Si-(OH)-Al groups, respectively, gallium loaded samples show additional IR band at 3699 cm(-1) which has been attributed to OH groups attached to Ga, such as Ga-2(OH)(x)O3-x. Dose-dependent measurements have shown that upon benzene loading of 1.7 molecules/u.c. (m/u.c.) for metal modified H-ZSM-5, the reagent molecules are located in the channel intersection. Above 1.7 m/u.c., part of benzene molecules enter the straight or sinusoidal channels and form pseudo liquid phase. The formation of pseudo liquid phase begins at low benzene loading for H-ZSM-5 zeolite. With increasing concentration of benzene molecules in zeolite channels, each molecule undergoes perturbation from the surrounding molecules and framework, which leads to a modification in electronic state and in the dipole moment of different vibrations. Abnormal adsorption isotherms for pure zeolite and gallium loaded samples have been correlated with this modification. This effect is not observed for Pt loaded samples, probably due to possible adsorption of benzene molecules on platinum particles dispersed on the external surface of zeolite crystals. The present paper provides important information on the effect of pre-treatment on the state of active species and the confinement and interaction of benzene molecules in a series of modified H-ZSM-5 catalysts. (C) 2003 Elsevier Science B.V. All rights reserved.