The isomerization of a-terpineol (1) catalyzed by heteropoly acid H3PW12O40 (PW) in homogeneous and heterogeneous systems yields 1,8-cineole (2) and 1,4-cineole (3), both useful for flavoring and pharmaceutical applications. In the homogeneous system, 2 and 3 were obtained with 25% and 23-27% selectivity, respectively, at 50-90% alpha-terpineol conversion (in a nitrobenzene solution, 40 degrees C). In the heterogeneous system, 35% of 2 and 25% of 3 were obtained at 70-100% conversion in a cyclohexane solution at 60 degrees C using silica-supported PW as a solid acid catalyst, and the catalyst could be recycled. PW showed a higher catalytic activity and selectivity than conventional acid catalysts such as H2SO4 and Amberlyst-15. (c) 2006 Elsevier B.V. All rights reserved.