MSU-S((BEA)) and MSU-S((y)) mesoporous molecular sieves with different Si/Al ratios were prepared and characterized by XRD, XRF, N(2) adsorption, (27)Al MAS NMR, NH3-TPD and 2,6-di-tert-butyl-pyridine adsorption. Their catalytic behavior for the liquid phase isomerization of alpha-pinene has been investigated and compared with conventional zeolites and mesoporous molecular sieves. The activity correlates well with the amount of the accessible acid sites on the catalyst surface. MSU-S((BEA)) with Si/Al ratio of 67 has the highest activity in comparison to others. Ninty-seven percent conversion of alpha-pinene and 91% yield for main products like camphene, limonene, tricylene and terpinolene can be obtained at 70 degrees C. The catalyst is stable and reusable, and the product yield is only reduced by 10% after four runs, which is probably caused by the slow dealumination in the framework wall during the reaction. (C) 2010 Elsevier Ltd. All rights reserved.