Electrodeposition of zinc (Zn) from a strongly alkaline Zn(II) solution showed a potential oscillation in the region of the current density exceeding the diffusion-limited one, accompanied by the formation of dendrites with a stacked wafer structure. Scanning electron microscopic inspection showed that the dendrites changed their shape periodically in synchronization with the potential oscillation from thick hexagonal wafers on the negative potential side to thin maple-leaf-like wafers on the positive potential side and inversely. In situ electrochemical quartz crystal microbalance measurements revealed that the rate of Zn deposition also oscillated in synchronization with the oscillation. A mechanism is proposed to explain the oscillation and the dendrite formation by taking into account coupling of autocatalytic Zn-crystal growth, in close relation with Mullins-Sekerka instability, and autocatalytic surface oxidation at the close packed crystal faces leading to surface passivation. (c) 2004 The Electrochemical Society. All rights reserved.