The Nernst-Planck equations of electrodiffusion have been reconsidered to encompass the treatment of asymmetric media. A model is proposed for the calculation of the electromotive force (emf) as well as the electric field profile and the electric potential profile in asymmetric membranes with any distribution of the single independent concentration in the case of two-ion systems. The model is used to illuminate the initial time emf method recently proposed to investigate the surface layers of asymmetric ion exchange membranes. The calculations demonstrate that this method is working as intended using a typical cellulose acetate membrane as an example. We also give some examples of calculations with concentration profiles extending over the full width of the asymmetric membrane, where no theoretical shortcuts are possible and the emf is truly a functional of the concentration profile.