in electrochemistry, the difference between the equilibrium (with respect to the composition of the electrode solution) value of electrode potential and the value of the electrode potential when the electrode is polarized by an external current. When the departure from equilibrium is noticeable, the relation between the overvoltage η and the density i of the polarizing current is usually given by the Tafel equation η = a + b log₁₀i where a and b are empirical constants. Overvoltage depends on the temperature, the nature of the electrode material, and the composition of the solution.

Overvoltage is necessary for the acceleration of a desired electrode reaction. If the rate of the electrode reaction is determined by the rate of the electrochemical stage proper, which involves charge transfer, then overvoltage amplifies the electric field acting on the particles undergoing discharge, with a resulting lower discharge activation energy: Since the electric field is to a great extent due to the structure of the electrical double layer, overvoltage turns out to be dependent on the concentration of the foreign electrolyte and the adsorbed materials, which affect the distribution of potential in the double layer.

Overvoltage has some positive apects; for example, the action of many inhibitors of metal corrosion is based on an increase in overvoltage. At the same time, overvoltage in industrial electrolysis inevitably involves additional power consumption and leads to increased production costs.