Acidic solid catalysts with different types of acidity were used to study the liquid-phase alkylation of phenol with isobutene. A phosphonium ionic liquid immobilized on silica type carrier exhibiting pure Lewis acidity, Amberlyst 15 with pure Bronsted acidity as well as WO3/ZrO2 with both types of acid sites were used for this study. The active sites are postulated based on pyridine-FT-IR and NH3-TPD studies, BET analyses, MAS NMR and XRD measurements. The different properties of the chosen catalysts are mirrored in the product distribution of the reaction mixture. It was found that WO3/ZrO2 is a very active and selective catalyst for the production of 2,4-di-tert-butylphenol under mild reaction conditions. (C) 2008 Elsevier Inc. All rights reserved.