The main effects of oxygen vacancy defects on the electronic and optical properties of V 2 O 5 nanowires were studied through in-situ Raman, photoluminescence, absorption, and photoemission spectroscopy. Both thermal reduction and electrochemical reduction via lithium insertion leads to the creation of oxygen vacancy defects in the crystal that gives rise to new electronic mid-gap defect states at energy 0.75 eV below the conduction band edge. The defect formation results in delocalization and injection of excess electrons into the conduction band, as opposed to localized electron injection as previously suggested. Contrary to what is seen in most oxides, the presence of vacancy defects leads to band filling and an increase in the optical band gap of V 2 O 5 from 1.95 eV to 2.45 eV, which is attributed to the Burstein-Moss effect. Other observed changes in the optical properties are correlated to the changes in the electronic structure of the oxide as result of defect formation. Further, in-situ Raman measurements during the electrochemical reduction at room temperature show that the oxygen atom that is most readily reduced is the three-fold coordinated oxygen (O3). Reduction of V 2 O 5 through the formation of oxygen vacancy, V O , defects have been studied both by theoretical and experimental studies. 10,11,12,13,14,15 In this work, we address some of the key unanswered questions regarding the correlation between optical properties and electronic structure of The rise of absorbance in the NIR spectral range in reduced oxide is a result of optical excitation of V O defect related mid-gap states. In addition, using in-situ Raman measurements during reduction process, we show that the oxygen atom that most readily participates during electrochemical reduction at room-temperature and gets reduced is the three-fold coordinated oxygen (O3).Vacancies in vanadium oxide have been probed using a variety of spectroscopic techniques, such as scanning tunneling microscopy, 9,26,27 electron energy loss spectroscopy, 28,29 electron paramagnetic resonance studies, 16,12,23 X-ray absorption measurements, 30 and X-ray photoemission spectroscopy (XPS). 15,17 However, most of these techniques either require ultrahigh vacuum (UHV) or controlled environment for operation that makes it difficult to adapt it to 6 in-situ catalytic studies. Here, we use near-infrared photoluminescence (NIR-PL) and Raman spectroscopy that allows for study of vacancy defects and their interaction with redox species under in-operando electrochemical conditions at room temperature, as shown in our recent study. 31 The spectral range of various types of electronic transitions in V 2 O 5 is shown in Fig.1A.Representative PL spectra of stoichiometric and non-stoichiometric V 2 O 5 are shown in Fig.1B.The optical gap of nominally undoped V 2 O 5 is in the range of 1.9-2.5 eV and hence emission spectrum lies in the ultraviolet-to-visible part of the spectral range. In most TMOs, deviation from stoichiometry is a result of the presence of high density ...