Abstract: Electroreduction of eriochrome blue black B was studied by d.c. pola-rography, differential pulse polarography and cyclic voltammetry. Only asingle wave which was diffusion-controlled and totally irreversible appearedover the range of pH2-12. With increasing pH, the electron number(according to logarithmic analysis) of the electrode reaction was changedfrom 2 to 4 around pH6, and the E_1/2 shifted linearly toward more nega-tive potential with a slope of -65mV/pH. Based on the experimentalresults, a mechanism involving the disproportionation of an unstable inter-mediate was proposed for the reducton of eriochrome blue black B, andby this, the change of electron number with pH was explained.