A series of sulphate-promoted ZrO2 solid acid catalysts with different contents of SO42- were calcined at 450 degrees C in air for 4 h and tested for the liquid-phase alkylation of catechol to guaiacol in a fixed-bed down-flow reactor. The 5 wt.% SO42- on ZrO2 showed the best conversion (82%) and selectivity for guaiacol (84%) at 200 degrees C and 1 bar pressure. A smooth correlation was observed between the catalytical activity and surface acidity of sulphated zirconia. Based on our results, a surface mechanism is proposed.