The evolution of structural and optical properties by V substitution in magnetite (Fe 3 O 4 ) was investigated. V x Fe 3−x O 4 (x≤1.0) compounds maintain a cubic structure of magnetite with V ions mainly having a valence of +3 and occupying octahedral sites, revealed by using X-ray diffraction, X-ray photoelectron spectroscopy, and Mössbauer spectroscopy. Optical constants of V x Fe 3−x O 4 thin films measured by spectroscopic ellipsometry in the 1–8 eV range vary significantly with V composition. The octahedral Fe ions are interpreted as being responsible for the observed reduction in the strength of the major absorption structures of Fe 3 O 4 by V doping. Thus, a number of charge-transfer transitions involving octahedral (eg and t2g) and tetrahedral (t2 and e) Fe d states are identified as contributing to the optical absorption spectrum of Fe 3 O 4 and their energy differences are estimated.