The thermal behavior in oxygen of Fe3-xVxO4 spinels (0 < x < 1) prepared by the ceramic method has been investigated over the temperature range 150-650 degrees C by thermogravimetry and infrared spectroscopy. Below 500 degrees C, iron vanadium spinels are partly oxidized in cation-deficient spinels with a large content of vacancies. A quantitative analysis based on the difference in the reactivity of iron and vanadium in relation to the site occupied (octahedral B site or tetrahedral A site) permits a cation distribution between the sub-lattices to be proposed. Kinetic studies of the oxidation process of V3+ ions at the B sites show that the oxidation reaction is controlled by cation diffusion in a cation-deficient spinel phase of variable composition, with a chemical diffusion coefficient dependent on the vacancy content delta and written : ($) over tilde D = K-0 exp(-E(a)/RT) exp(-b delta)). The b constant depends on the temperature. The energy of defect dissociation has been determined.