The reaction between a hydroquinone aqueous solution and manganese dioxide has been investigated by electron paramagnetic resonance spectroscopy. A very stable p‐benzosemiquinone anion is formed when an aqueous solution of hydroquinone is passed through a column of manganese dioxide. The kinetics for the reaction between hydroquinone and manganese dioxide follow the equation (Formula Presented.) where A0 is the initial concentration of hydroquinone, C is the concentration of p‐benzosemiquinone anions, W is the amount of manganese dioxide, F is the feed rate of a hydroquinone aqueous solution, and k10 and k20 are constants; k10 = 3.1 × 10−2 (cm3/g MnO2· min), k20 = 3.2 (cm3/g MnO2·min). A mechanism for the reaction is proposed such that Mn(IV) in manganese dioxide is reduced to MnII with hydroquinone from which the semiquinone anion is formed via a neutral semiquinone. The formation of semiquinone anions from several substituted hydroquinones is also discussed.