tert-Butylation of phenol and cresols using tert-butyl alcohol as an alkylating agent was studied in the presence of Fe-modified montmorillonite K10 catalysts in liquid phase. The catalysts were prepared by treating montmorillonite K10 with aqueous and acetonitrile (organic) solution of FeCl3. The present study demonstrates the potential of K10 catalysts in the production of tert-butylphenol with high activity and selectivity. One hundred percent tert-butylation was observed in all the catalysts. Regioselectivity in the reaction is also studied. (C) 2003 Elsevier B.V. All rights reserved.