Abstract Kinetics and mechanism of the liquid-phase reaction, 2,6-di-t-butyl-4-methylphenol (DBMP) + ArH → 2-t-butyl-4-methylphenol + ArBu t, in the presence of bulk and SiO 2-supported heteropoly acid H 3PW 12O 40 (PW) as a heterogeneous catalyst have been studied (ArH = PhH, PhMe, PhBu t, o-xylene, m-xylene, [DBMP]/[ArH] = 1 22 mol mol −1, 5–25 °C). Under these conditions no isobutene is formed. The reaction was found to be first order with respect to both DBMP and the catalyst. The rate depends on the nature of ArH in a way that is typical of electrophilic aromatic substitution. The activation energies are 29 ± 3 and 19 ± 2 kcal mol −1 for bulk PW and 25% PW/SiO 2, respectively. The activity of the surface protons of PW/SiO 2 (turnover number) increases monotonically as their acid strength increases with increasing total PW loading, reaching a maximum for the bulk PW. The mechanism of the reaction is discussed.