Solid phosphoric acid (SPA), silicated amorphous silica-alumina (ASA) catalysts (1.5-70% SiO2), and zeolites beta (BEA), mordenite (MOR), and ZSM-5 (MFI) were investigated for the alkylation of phenol with I -octene. All catalysts were active for phenol alkylation at 200 degrees C in batch reactors, with olefin oligomerization being the main side reaction. All catalysts produced alkylphenols and phenyl ethers and had a low initial selectivity to o-octylphenol. Monoalkylated product selectivity on silicated ASA may be related to the orientating effect of the silicated catalyst surface but could not be related to the acid strength or alumina content of the catalysts. Evidence is also presented that indicates that an ester-based mechanism is active over silicated ASA. Monoalkylated product selectivity on zeolites could be correlated to the average pore size of the catalysts (shape selectivity). Multiple alkylation was found on SPA, Siral-28M, and the zeolite catalysts, with both acid strength and pore size determining whether multiple alkylation occurred.