The Nernstian plots in a spectropotentiostatic experiment for complex electrode processes were studied theoretically. The plots are linear for an electron transfer process coupled with a preceding or succeeding chemical reaction, and the electron stoichiometry can be determined from the slope. The intercept on the potential axis is dependent on the equilibrium constant of the chemical reaction. For the EE and ECE mechanisms, a single redox site contains multiple redox states and the Nernstian plot is non-linear. However, the curve asymptotically approaches two straight lines at very positive and very negative potentials. The electron stoichiometry and the formal potential can be estimated from the slope and intercept of the asymptote.