Nigella sativa has been used for centuries as a natural remedy for a number of chronic and age-related diseases. Thymoquinone (TQ), the main constituent of its extracts, has recently received particular attention and has been tested for its antioxidant, anti-inflammatory and anticancer properties. To further investigate the mechanisms involved in the biological activities of this natural quinone and, among these, in its antioxidant properties, the redox-system of TQ and its interaction with superoxide was studied in aprotic medium by cyclic voltammetry, spectroelectrochemistry and Electron Spin Resonance (ESR). The electrochemical behavior of dithymoquinone (DTQ), the photodimer of TQ was also studied in the same medium. Experimental data evidenced the formation of the radical anion TQ by cathodic reduction of TQ at potential values very close to coenzymes Q ones, by electron transfer (ET) between TQ and superoxide, as KO2 or electrogenerated, by chemical comproportionation between TQ and the dianion TQ(--) and by fast cleavage of the electrogenerated radical anion DTQ(center dot-). Spectroelectrochemical data evidenced that TQ(center dot-), in the presence of TQ evolves to the hydroquinone monoanion TQH(-), suggesting that an H-atom transfer (HT) may occur, likely from the isopropylic side-chain of TQ to TQ(center dot-) The H-atom donating ability of TQ may be also supported by Bond Dissociation Energy values and ESR data. (C) 2011 Elsevier Ltd. All rights reserved.