Anodisation of a Nb-Zr alloy was studied in ammonium sulphate and hydrochloric acid electrolytes using electrochemical and surface analysis methods. Using electrochemical impedance spectroscopy (EIS) and linear sweep voltammetry, a value of epsilon(tau) = 27 was estimated for the relative permittivity and of 0.4 V nm(-1) for the electrical field in the oxide film. Annealing of a mechanically polished disc sample to 600 degreesC for 14 h in UHV resulted in a fourfold surface enrichment in zirconium. This enrichment remained after anodisation in the sulphate solution, but could be removed with an anodisation in hydrochloric acid. Auger electron spectroscopy was used to estimate the thickness and composition of the anodic oxides. The film thickness measurements suggested a logarithmic growth law. The surface concentration of zirconium at the outer film surface of heat-treated samples decreased logarithmically when anodisation was performed in 1 M hydrochloric acid. No corresponding decrease in zirconium concentration was observed during immersion at open circuit. (C) 2003 Elsevier Ltd. All rights reserved.