[en] An analysis on mass transfer losses, or concentration overpotentials in fuel cells is provided. A simplified theory, as proposed in some texts, is derived. This is followed by a more rigorous theoretical foundation of mass transfer, for which the concentration overpotential is obtained as a function of driving force. The solution for the driving force is provided, for the well-known one-dimensional convection-diffusion problem. The methodology is illustrated using the simple example of a solid oxide fuel cell. It is shown that for reduction of O₂ at the cathode, the simplified theory is only applicable for very dilute mixtures. At the anode, where H₂ is consumed and H₂O produced, the simplified theory fails. A linearised theory, appropriate for low mass flow rates is also presented. (author)