|Measuring the concentration of chloride ions in concrete cover is important for the maintenance of concrete structures under severe conditions, such as those found in a coastal environment. This paper describes an on-site method of using Ag/AgCl microelectrodes to estimate chloride ion ingress depth into concrete structures. Because the microelectrode drill-holes need a diameter of only ~10 mm, the measurements cause little damage. The electrode potential when the edge of the microelectrode is in contact with the concrete cover reveals the extent of chloride ion ingress, regardless of cement type and water-cement ratio. This method has been used to detect high concentrations of chloride ions (more than ~2.4 kg/m3) in chloride-penetrated members removed from an actual pre-stressed concrete bridge.