An O-alkylation reaction of hydroquinone with excess methanol was carried out by using alkaline metal ion-exchanged zeolite catalysts at various phases of methanol in a slurry type reactor. The amount of methanol, used as a methylating agent and also a solvent, significantly affected the reaction rate. When the amount of methanol was decreased from 2 mol to 0.6 mol, the reaction rate was increased more than nine times. These changes in the reaction rate could be explained by the pressure change and the phase transition of the reactant, methanol, depending on the temperature and the elimination of a diffusion limitation of reactants through the zeolite pores in a gas phase condition. Thus, higher than 89% selectivity to 4-methoxyphenol was obtained at 80% conversion of hydroquinone at a gas phase reaction condition using 1 mol of methanol for 2 hours.