The intermediates of the 1,4,11,12-tetrahydro-9,10-anthraquinone (THAQ) to 9,10-anthraquinone (AQ) reaction are studied by various spectroscopic and computational methods. X-ray diffraction, two-dimensional nuclear magnetic resonance (NMR) spectroscopy, and geometry optimization calculations, by the UB1LYP hybrid density functional technique, show that THAQ initially exists in the keto form (cisTHAQK). Addition of very small amounts of NaOH to cisTHAQK in solution catalytically converts it into the corresponding enol form (THAQE). The THAQE is then oxidized, by dissolved O-2 in solution, to give the novel 1,4-dihydro9,10-anthraquinone (DHAQ) which is isolated, and its single-crystal structure is characterized by X-ray spectroscopy. If NaOH is added to THAQE, the 1,4-dihydro-9,10-anthraserniquinone (DHASQ) radical anion is produced. It is detected by electron paramagnetic resonance (EPR) spectroscopy, and its experimental nuclear hyperfine coupling constants are correlated with those computed by the UB1LYP method. Consequently, the production of DHASQ radical anions, upon addition of NaOH, follows the stepwise reaction cisTHAQK reversible arrow THAQE = DHASQ. The treatment of DHAQ with NaOH in methanol also generates the DHASQ radical proving that DHAQ is a precursor to DHASQ. It is thus shown that the DHASQ radical anion can be generated either by oxidation of the THAQE or the single electron reduction of the DHAQ. When the DHASQ radical anion is exposed to small amounts of O-2, it is converted into the corresponding 9,10-anthrasemiquinone (ASQ). Further O-2 oxidizes this radical anion to AQ. This proves that DHASQ must first form the ASQ intermediate before being converted to AQ. These results reveal that the THAQ-soda pulping process, in the presence of atmospheric O-2, first produces AQ. From then onward it is the same as the popular AQ-soda pulping process used in the paper manufacturing industry.