Two redox processes of methyl viologen (+2/+, +/0) in acetonitrile were investigated by using channel electrode voltammetric and in situ electrochemical ESR methods. Two separated unequal plateau currents of the first (+2/+) and second (+/0) redox processes of the viologen were observed in the channel electrode voltammograms and showed a cube-root depedndence on the electrolyte flow rate, respectively. The simple Levich analysis resulted in two different diffusion coefficients of $D_{+2}=2.2{ imes}10^{-5};cm^2/s$ and $D_+=3.0{ imes}10^{-5}cm^2/s$ from the limiting currents. In situ electrochemical ESR studies were performed for the monocation radicals generated at the potentials of the two plateau currents in the electrolyte flow range $1.3{ imes}10^{-1}{geq}v_f{geq}2.7{ imes}10^{-3};cm^3/s$. Backward implicitfinite difference method was employed to simulate the electrochemical kinetic problem of two sequential electron transfers ($MV^{+2}+e{leftrightarrows}MV^+,;MV^{+}+e{leftrightarrows}MV^0$) coupled with reversible comproportionation ($MV^{2+}+MV^0{{leftrightarrows}^{k_f}_{k_b}}2MV^+$). $k_f$ was found to be greater than (<TEX>$10^6M^{-1}s^{-1}.