The relative degradation rates of metallic materials in aqueous, copper-based timber preservatives have been investigated using electrochemical impedance spectroscopy. The metals examined were AS/NZS 1595: 1998, grade CA1 mild steel, AISI 316 stainless steel (UNS S31600) and AS 1397, grade Z275 hot-dipped galvanised coil-coated sheet. The electrolytes consisted of commercially sourced copper-chrome-arsenate, copper azole and alkaline copper quaternary electrolytes diluted to a concentration of 0.1 mol dm(-3) copper. The electrochemical impedance response of each system has been modelled and the resulting relative rates of corrosion have been shown to broadly correspond to general trends taken from previous weight loss and direct current studies incorporating similar electrolytes. The hot-dipped galvanised steel corroded in an active manner at rates that were two to four orders of magnitude greater than that of the ferrous-based materials; the latter appeared to passivate in all instances.