One-electron reduction of dihydroxyanthraquinone derivatives was studied in dimethylsulfoxide solution by means of electrochemical and spectrophotometric methods. It was found that dihydroxyanthraquinone reduction in the presence of oxygen occurs according to an EC mechanism, the second step being oxygen reaction with semiquinone anion radical. This mechanism was confirmed by the dependence of CV curves on dihydroxyanthraquinone and oxygen concentration. Supplementary spectrophotometric and electrochemical studies of the dihydroxyanthraquinone-potassium superoxide system have shown that no covalent compound is formed between seminquinone anion radical and molecular oxygen and therefore the reaction following dihydroxyanthraquinone reduction involves electron and proton transfer from semiquinone anion radical to molecular oxygen to form hydroperoxyl radical and deprotonated dihydroxyanthraquinone. The rate constants of the electron-transfer reactions from semiquinone anion radical to molecular oxygen were determined from the dependence of anthraquinone cathodic reduction peak potential and the corresponding anodic peak current on oxygen concentration. It was found that the rate constants increase with decreasing pK(a) of the hydroxyanthraquinones, which confirms the postulated mechanism.