The preparation and characterization of the 12-niobomanganate(IV) heteropolyanion is described. This complex appears to be the first heteropolyniobate in which discrete anions can be discerned. The electronic absorption and diffuse reflectance spectra of the anion are reported, as are its magnetic moment and e.s.r. spectrum. Attempts to prepare a corresponding heteropolytantalate were unsuccessful, but a mixed heteropoly-complex, containing both niobium and tantalum, has been prepared.