Methyl viologen (MV) is an electron mediator that has been shown to be beneficial for enhancing product formation in biofuel processes. For example, increased yields have been observed for ethanol and butanol production. MV has also been used in bioremediation processes such as removal of pollutants from groundwater. However, MV has also been shown to have detrimental effects including inhibition of cell growth. Unfortunately, studies have shown little information regarding the MV redox state. A thermodynamic model is presented and validated in order to predict the most likely redox state of MV depending on the redox potential, pH, and degree of MV dimerization. Model results showed that there are narrow potential ranges in which MV can change from one redox state to another. During fermentation processes in which the potential is in these narrow ranges, slight perturbations in the redox potentials can drastically change the redox state of MV. This may explain the observed variations in the positive and negative effects of MV. Thus, caution should be used when applying MV to any biological system. If needed, an electrode system may prove useful for controlling the redox potential and the associated MV redox state.