The width of the electrochemically active zone in a mixed conducting solid oxide fuel cell (SOFC) cathode, i.e. the spatial extension of the three-phase-boundary (3PB) region is numerically calculated for the case when oxide ion transport through the electrode bulk determines the reaction rate of the cathodic oxygen reduction reaction. The current density distribution in the cathode exhibits a sharp maximum at the 3PB and a major fraction of the total current flows in narrow zones with a width of a few percent of the electrode particle size (e.g. in the case of 1.6 mum particle size: approximately 2/3 of the current flows in a zone with a width of 60 nm). Hence, the corresponding polarisation resistance is almost inversely proportional to the 3PB length. In a wide parameter range an increasing ionic conductivity of the mixed conductor does not broaden the electrochemically active region. A formula is presented which allows an estimate of the polarisation resistance of a mixed conducting electrode if ionic transport through the electrode bulk is rate limiting.