High-silica offretite (Si/Al = 7.6-8.0) with no or very little amount of erionite was successfully obtained by the interzeolite conversion of faujasite used as a starting material in the presence of both benzyltrimethylammonium hydroxide and the structure-forming alkali metal cations such as Li(+) and Na(+). Hydrothermal conversion of faujasite into offretite depended on the Si/Al ratio of the starting faujasite. Only faujasite with Si/Al ratio of 16-31 was converted into offretite. In this conversion, offretite was obtained in 1 day, indicating that the crystallization rate was enhanced by using faujasite as the starting material. Based on (13)C CP/MAS NMR measurements, most of benzyltrimethylammonium cations were presented intact within zeolitic pores of the obtained offretite. As offretite was not obtained from amorphous materials (amorphous SiO(2) and gamma-Al(2)O(3) or Al(OH)(3)), the advantage of the interzeolite conversion was demonstrated. (C) 2010 Elsevier Ltd. All rights reserved.