A new general equation is derived for the overpotential of a polarizable, electroneutral, isothermal electrode surface using nonequilibrium thermodynamics for surfaces. Local equilibrium at the surface is assumed. The use of independent variables allows a separation of fast and slow processes and a simple incorporation of the chemical reaction into a set of coupled scalar fluxes. The theory is illustrated by the hydrogen evolution reaction in the steady state. This reaction can be regarded as the coupled transport of mass fluxes with the electric current density. The overpotential, which is a singularity of the electric field at the surface, can be described entirely in terms of linear flux-force relationships. It is explicitely shown how the formulation includes different versions of the empirical Tafel equation and how parameters of the Tafel equation may be reinterpreted. This gives a completely new basis for describing and understanding the overpotential of an electrode. The value 0.5 of the so-called symmetry factor a, is in our theory a consequence of stoichiometric relationships and special transport coefficients at the surface, not, e.g., of the position of the activation energy barrier. Deviations from 0.5 can be explained by interacting fluxes. Such interactions are dependent on the current density and on the temperature, and eventually may lead to a shift of rate-limiting steps. The inclusion of coupling phenomena into the theory for the overpotential may resolve observations quoted in the literature as anomalies. The overpotential in the steady state can be obtained as a function of several transport coefficients for the surface in addition to the ohmic surface resistance. A negative slope of ln j with partial gas pressure of hydrogen is predicted.