ESR studies of unsupported V2O5-Fe2O3 catalysts revealed that a new phase is formed by doping the solids with cesium sulfate. The percentage of this phase increases with increasing iron content. As indicated by XRD and Mossbauer spectra, the new phase is amorphous. Its composition is nonstoichiometric and varies slightly depending on the iron content. For catalysts with an atomic ratio of V:Fe:Cs = 1:1:0.06 and 1:1.4:0.06, approximate formal compositions of FeO.70VO3.O4 and FeO.65VO3.14, respectively, were calculated, The new phase contains Fe3+ ions with oxygen lattice vacancies in their coordination sphere being occupied by an electron. Since the 9-fluorenone selectivity is improved drastically on cesium doping and with increasing iron content it seems likely that in addition to the diminished surface acidity these vacancies serve as active centres in the catalytic gas phase oxidation of fluorene.