Oxide layers were grown on tubular samples of Zr-1%Nb under conditions simulating those in VVER-type pressurised water reactors, viz. in near-neutral borate solutions in an autoclave at 290 o C. These samples were investigated using electrochemical impedance spectroscopy which was found to be suitable to follow in situ the corrosion process. A -CPE o x R o x - element was used to characterise the oxide layer on Zr-1%Nb. Both the CPE o x coefficient, σ o x , and the parallel resistance, R o x , were found to be thickness dependent. The layer thickness, however, can only be calculated after a calibration procedure. The temperature dependence of the CPE o x element was also found to be anomalous while the temperature dependence of R o x indicates that the oxide layer has semiconductor properties. The relaxation time - defined as (R o x σ o x ) 1 / α - was found to be quasi-independent of oxidation time and temperature; thus it is characteristic to the oxide layer on Zr-1%Nb.