The reduction of anthraquinone (AQ) with alkaline solutions of sodium sulfide, which takes place in the kraft pulping process, has been studied. The reduction product is the disodium salt of 9,10-dihydroxyanthracene (Na(2)AQ). A 2(4) factorial design has been used to check the effects of sodium sulfide, anthraquinone and sodium hydroxide concentrations and temperature on the reaction. From statistical results, it has been concluded that the concentration of reducing agent has a limiting effect on Na(2)AQ yield, while the temperature has a negative effect because it favours the oxidation of sodium sulfide in alkaline solutions. Therefore, a maximum reduction of quinone can be obtained with the highest concentration of reagents and low temperatures.