The structureโactivity relationship and reaction mechanism for selective oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF) in toluene were studied on vanadium oxide domains on TiO~2~, Al~2~O~3~, Nb~2~O~5~, ZrO~2~, and MgO and with a wide range of VO~x~ surface densities. The structures of these catalysts were characterized by X-ray diffraction (XRD), diffuse reflectance UVโvis spectroscopy (UVโvis DRS), and Raman spectroscopy, and their reducibility was probed by H~2~-temperature programmed reduction. The structures of the VO~x~ domains evolved from monovanadate to polyvanadate structures with increasing the VO~x~ surface densities, and finally to crystalline V~2~O~5~ clusters at surface densities above one-monolayer capacity. Within one-monolayer capacity, higher VO~x~ surface densities and more reducible supports led to higher reducibility and reactivity of the VO~x~ domains. The support surfaces covered with polyvanadates and V~2~O~5~ clusters and the supports with acidity favored the formation of DFF. The correlation between the reducibility and reactivity, together with the kinetic studies, suggests that the HMF oxidation to DFF proceeds via the redox mechanism involving the V^5+^/V^4+^ redox cycles and the reoxidation of V^4+^ to V^5+^ by O~2~ as the rate-determining step. These results may provide guidance for the design of more efficient catalysts for the HMF oxidation to synthesize DFF.