Nitro Blue Tetrazolium (NBT) is a widely used chemical for the determination of superoxide anion radical (O-2(-)) and superoxide dismutase activity in many fundamental biological processes. Its reduction reaction with superoxide and the associated spectral absorption data of its reduction products, i,e., monoformazan (MF) and diformazan (DF), were studied in the Current research. The molar extinction coefficient, epsilon, of each of these two reduction products in dimethyl sulfoxide (DMSO) were calculated to be 120 000 and 235 000 M-1 cm(-1), respectively. We propose quantifying these species as a means of obtaining superoxide anion radical concentration or as a general screening protocol for the efficacy of an antioxidant.