Liquid phase catalytic isomerisation of alpha-pinene with natural Indian Montmorillonite modified by sulphuric acid treatment and cation exchange was studied. More than 96% alpha-pinene conversion is observed for acid treated clays with camphene selectivity ranging from 39 to 49%. For acid treated clays prepared by treating with low normality acid (1-4N), limonene is the main product whereas clay samples prepared with high normality (5-9N) acid, alpha-terpinene is a main product among monocyclic terpenes. Ce3+, Fe3+, La3+, Ag+, exchanged clays showed higher than 90% conversion for alpha-pinene with 39-119% selectivity for camphene. However, Li+, Ca2+, Mg2+ exchanged clay showed poor conversion (4-12%) for alpha-pinene. Observed conversions and selectivities have been explained in terms of surface acidity, structural and textural features of the modified clays determined by using X-ray diffraction and N-2 adsorption at 77 K. Correlation of alpha-pinene conversion with cyclohexanol dehydration showed that alpha-pinene conversion is a Bronsted acid catalyzed reaction. (C) 2004 Elsevier B.V. All rights reserved.