The anodes used in SOFCs are composites, formed by a mixture of nickel and YSZ particles. This paper presents a model for this type of electrode, taking mass transport effects into account. The effect of the operating conditions, such as temperature and pressure, is discussed. Also, the effect of the choice of the geometrical parameters, such as electrode thickness and particle radius, on the electrode performance is analysed in detail. In particular, the electrode losses display a minimum for a well-defined radius of the electrode particles, which is related to a trade-off between activation and concentration losses.