To provide a process for producing trimethylhydroquinone in relatively high selectivity from hydroquinone and methanol while generating little tetramethylhydroquinone as by-product.

Hydroquinone is made to react with methanol in the absence of catalyst under a condition to form supercritical state of methanol. A catalyst is added to the reaction system and reaction is further continued under a condition to form supercritical state of methanol. Carbon dioxide is included in the supercritical methanol and the reaction is carried out under a condition to form a supercritical state of the mixture of methanol and carbon dioxide.