This paper describes our investigations by E.P.R. technique of the paramagnetic centres formed during the reduction at high temperature in vacuum of MoO₃, WO₃, Ta₂O₅, Nb₂O₅. For all oxides we observed a signal at g = 0.002 both at -17O°C and at room temperature. This signal was destroyed reversibly by the presence of molecular oxygen; furthermore, it was not possible to observe any peroxidic species. The data have been discussed in view of the hypothesis that the paramagnetic centre is centre F or species O^-. Moreover, the E.P.R. spectra of the studied oxides with metaloxygen double bond were discussed in relation to those of oxides without such a metal-oxygen bond.