An improved process for producing hydroquinone from p-isopropenyl phenol is disclosed. Hydroquinone is obtained by oxidizing p-isopropenyl phenol with hydrogen peroxide or an organic peroxide as an oxidizing agent at a temperature of 28.degree. to 50.degree. C. in the presence of a strong acid catalyst using a slightly or hardly water-soluble high-boiling polar solvent. In the oxidation reaction, the mole ratio of the oxidizing agent to p-isopropenyl phenol is kept at less than 1. After the reaction, in order to effectively decompose or consume the undecomposed p-isopropenyl phenol hydroperoxide and the unreacted oxidizing agent remaining in the reaction mixture, the reaction mixture is aged. Alternatively, or subsequently to the aging reaction, SO.sub.2 is added to the reaction mixture to completely decompose the undecomposed or unreacted peroxide. Thus, hydroquinone of high purity is obtained.