The results of X-ray fluorescence measurements of niobium metal and its oxides (NbO, NbO2 and Nb2O5) are presented. Non-resonant Nb M4,5 and OKα X-ray emission spectra (XES) have been measured at the Advanced Light Source (ALS, Berkeley). The experiments are compared with self-consistent band structure calculations of niobium metal and niobium oxides. We find the experimental spectra are consistent with band structure calculations of niobium metal and the oxides NbO2 and Nb2O5. The difference between experimentally determined Nb M4,5 XES of NbO and the calculated spectrum of Nb1.0O1.0 is understood on the basis of vacancies. We have mimicked the natural structure of NbO, which contains vacancies amounting to 25% in both sublattices. The electronic DOS of Nb0.75O75 is characterised by the formation of additional subbands connected with vacancy states. The calculated electronic structure based on this model is consistent with the XES measurements. © 2002 Elsevier Science B.V. All rights reserved.