The Gerischer impedance in its most elementary form, Z(G)(omega)=Z(0)(k+jomega)(-1/2), has been observed in the frequency response of mixed conducting solid electrolyte systems. This simple transfer function can be derived directly from Fick's second law by including a reaction term. The Gerischer impedance was observed in the electrode dispersion of heavily Tb-doped yttria-stabilised zirconia (YSZ) ceramics. The mixed conducting samples were provided with ionically blocking gold electrodes. The occurrence of a Gerischer impedance in the Tb-YSZ system is tentatively modelled with the formation of immobile complexes of oxygen vacancies and trivalent cations. The presented results indicate that for mixed conducting materials it is not allowed to represent the electronic and ionic conduction path as independent components in an equivalent circuit.