Systematic electrochemical studies and scanning electron microscopy (SEM) observations were carried out on the anodic formation and cathodic reduction of dense AgCl (about 20 μm thick) on a planar silver substrate in 0.1 mol dm−3 HCl at room temperature. Special attention was given to the ionic conductivity of the AgCl deposit layer. In the major period of anodic growth of AgCl, partially solvated Ag+ ions in inter-grain microchannels were thought to be the main charge carrier. A model featuring periodical changes in the accessibility of microchannels to electrolyte solution was proposed to explain various phenomena, including overpotential fluctuation and complex changes of surface morphology. The initial stage of AgCl reduction was thought to be essentially the reverse process of AgCl growth. But the microchannels were soon broadened so that the electrolyte solution in channels became the main support of ionic conduction. The porous silver layer resulting from reduction of the dense AgCl had a porosity of 60%, which was determined by the difference in molar volume between AgCl and Ag. The average diameter d (μm) of Ag particles and the current density j (mA cm−2) of reduction were found to follow approximately a linear equation in the j range studied (0.2–3.2 mA cm−2): d=0.19−0.24log j.