3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide ([MTT]Br), which is of importance as a biological redox indicator, irreversibly adsorbs on a graphite electrode and exhibits two surface elechochemical redox processes. The mechanism of reduction and re-oxidation of this species on the surface is discussed as a function of pH. AM1 calculations and redox data are interpreted to show that protonation of the first reduced product leads to a ring-opening pathway, eventually generating the two-electron reduced formazan species. Cyclic voltammetry, rotating-disk voltammetry, pH dependence studies and electronic spectroscopy are used to characterize the system.