The addition of caesium sulphate to unsupported vanadium-iron-oxide materials leads to the formation of an amorphous non-stoichiometric phase, as well as the crystalline phases, FeVO4, Fe2V4O13, and Fe1-XS (pyrrhotite). Caesium was only present in the amorphous parts of the sample, and the amount of the new phase formed was found to vary depending on the vanadium : iron ratio and the amount of caesium dopant added. The new phase contains Fe3+ ions with oxide lattice vacancies in their coordination sphere, i.e. Fe3+-V-0(e(-)) species. Catalytic testing demonstrated that the caesium-doped samples were more selective for the oxidation of fluorene to 9-fluorenone. The role of the caesium is probably to modify the surface acidity of the catalyst and act in the formation of stable oxide lattice vacancies.