Mixed conductors-single phases that conduct electronically and ionically-enable stoichiometric variations in a material and, therefore, mass storage and redistribution, for example, in battery electrodes. We have considered how such properties may be achieved synergistically in solid two-phase systems, forming artificial mixed conductors. Previously investigated composites suffered from poor kinetics and did not allow for a clear determination of such stoichiometric variations. Here we show, using electrochemical and chemical methods, that a melt-processed composite of the 'super-ionic' conductor RbAg4I5 and the electronic conductor graphite exhibits both a remarkable silver excess and a silver deficiency, similar to those found in single-phase mixed conductors, even though such behaviour is not possible in the individual phases. Furthermore, the kinetics of silver uptake and release is very fast. Evaluating the upper limit set by interfacial ambipolar diffusion reveals chemical diffusion coefficients that are even higher than those achieved for sodium chloride in bulk liquid water. These results could potentially stimulate systematic research into powerful, even mesoscopic, artificial mixed conductors.