The technological feasibility of electrophoretic deposition (EPD) of gadolinium doped ceria (GDC) on polypyrrole (PPy) coated yttrium stabilized zirconia (YSZ) has been reported. To enhance the EPD technique for the fabrication of GDC layer and to deepen the understanding of EPD, the fundamental characteristics of EPD of GDC on PPy coated YSZ were investigated. The deposition rate of GDC on PPy coated YSZ is slower than that on graphite under the same voltage. An H+ ion accumulation zone is formed in the vicinity of PPy coated YSZ after applying the voltage. The thickness of this ion accumulation zone initially increases and then decreases. Finally, the ion accumulation zone is replaced by an ion depletion zone. The absorbed H+ ions are desorbed from the particles after deposition and are then reduced to hydrogen gas. The reduction of absorbed H+ ions and free H+ ions corresponds to the deposition and the unavoidable side reaction, respectively. A new parameter, f, the percentage of electric charged associated with the reduction of absorbed H+ ions in the total charge, is introduced to reflect the competitive relationship between the deposition and the inevitable side reaction. f decreases with the increase of current density. (C) 2019 Elsevier Inc. All rights reserved.