ZnO doping can significantly diminish the deleterious effect caused by impurities on the grain-boundary conduction of polycrystalline Ce_0.8Gd_0.2O_1.9 electrolyte. Analysis by field emission transmission electron microscopy equipped with energy-dispersive X-ray spectroscopy reveals that the siliceous and ZnO phases are separately aggregated at the triple grain junction areas in ZnO-added specimens. This finding implies that the scavenging process does not occur via the expected formation of zinc silicates, but via a novel mechanism in which resistive siliceous phases strongly aggregate in non-wetting configurations induced by ZnO. A three-dimensional schematic is established to elucidate the role of ZnO in the CeO₂–Gd₂O₃ system.