The changes in interface structure and chemical composition of a 99.995% pure nickel/yttria-stabilised zirconia (YSZ) interface were examined after heat treatment at 1000degreesC in 97% H-2/3% H2O with and without polarisation. Impedance spectroscopy was used for electrochemical characterisation. The results were compared to earlier investigations of a less pure nickel/YSZ interface.The pure interface developed different structures depending on whether or not the samples were polarised. Despite,the purity of the nickel, impurities were found in the interfacial region. The pure electrodes/interfaces showed, area specific polarisation resistances 10 times lower than the impure interfaces. (C) 2003 Elsevier Science B.V. All rights reserved.