Abstract The synthesis, and certain properties of manganese, cobalt, nickel, copper, zinc, and cadmium oxofluoroniobates and fluorotantalates, with compositions: MNbOF 5·4H 2O (M = Mn, Cd, Cu), MNbOF 5·6H 2O (M = Co, Ni, Zn), MTaF 7 · 6H 2O (M = Mn, Co, Ni, Zn, Cd) and CuTaF 7·4H 2O are described. The unit cell parameters of the substances synthesized were determined and the compounds MNbOF 5 · 6H 2O and MTaF 7 · 6H 2O shown to be isotypical; these crystallize in the hexagonal system, the diffraction class being 3 ̄ R or 3 ̄ mR , with three formula units in the unit cell. MnNbOF 5 · 4H 2O and CdNbOF 5 ·4H 2O crystallize in the orthorhombic system with four formula units per unit cell. The i.r. spectra of the substances synthesized are discussed.