X-ray absorption near-edge structure (XANES) spectroscopy is applied to Zr-based alloys ((Zr, x%Nb) with x ⩽ 1 wt.%) corroded in steam at 415 °C to resolve the niobium speciation in the metal and the oxide parts of their metal/oxide interface. By combining these synchrotron X-ray measurements with additional ab initio calculations of Nb K-edge XANES spectra, this study reveals (i) the presence of Nb0 in the metal and of Nb atoms having valency between +2 and +4 in the oxide as well as (ii) the coexistence of Nb-doped monoclinic and Nb-doped tetragonal zirconia structures within the oxide of the (Zr, 1.0%Nb) alloy.