Open circuit voltage is an important parameter of fuel cells. Prior works have demonstrated that open cell voltages of alkaline fuel cells fueled with glucose reach saturation at high glucose concentrations . At low concentrations, this voltage should increase logarithmically, according to the Nernst law. To study this reaction in the said fuel cells, open circuit voltages were measured over a wide concentration range. The fuel cell was operated as a continuous tank reactor undergoing a transient. During this transient, the concentration (either of glucose or KOH) of the solution in the fuel cell was decreased by several orders of magnitude. Measurements of voltage and concentration taken at different times tested their interdependence. Though no stirring was applied, the fuel cell behaves like a continuous stirred tank reactor. This was established by measuring concentration (either of glucose or KOH) versus time. The effect of concentration on the open circuit voltage was examined from down to for glucose and from to for KOH. The open cell voltage depends logarithmically on the glucose concentration at low glucose concentrations, up to . From the Nernst law, it may be deduced that one electron is transferred by one glucose molecule to the anode. The open cell voltage is constant, , at KOH concentrations from down to , dropping down to at KOH. Operating a fuel cell as a continuous stirred tank reactor is an efficient way of measuring fuel cell performance over a wide range of fuel and electrolyte concentrations. Analyzing the effect of concentration on cell voltage provides insight into the reaction mechanism.