Four kinds of large port zeolites BEA, FAU, LTL and MOR, were used as catalysts in the dehydration of a large molecule, epiandrosterone, to elucidate the effect of their pore structure on its conversion and the selectivity for 5 alpha-androst-2-en-17-one (Delta 2-olefin). Silane masking of the zeolite acid sites, measurement of their epiandrosterone uptake, and theoretical calculations to find the energy-minimized conformation in zeolite pores revealed the roles of pore structure and acid sites in the dehydration. LTL and MOR zeolites showed poor conversion and low selectivity for Delta 2-olefin because of the close packing in their linear pores. BEA and FALI zeolites with three-dimensionally connected pores exhibited high conversion. Since the supercages of FAU zeolite provided enough space to allow tilted conformation of epiandrosterone with high preference for the formation of Delta 2-olefin, FAU zeolites with a sufficient number of acid sites demonstrated both high conversion and selectivity in the dehydration of epiandrosterone. (C) 2009 Elsevier B.V. All rights reserved.