The possibility of forming spinel-type ferrites MFe2O4from M(OH)2or MO and α-FeO(OH) (goethite) or α-Fe2O3(hematite) under hydrothermal conditions up to 350° has been investigated experimentally for M = Mg, Ca, Mn, Co, Ni, Cu, and Zn. Formation of MFe2O4was observed for M = Mn (jacobsite), Co, Ni (trevorite), and Zn (franklinite), and was in general favored by high temperatures, high pH, and the presence of NaNO3or, especially, NaCl in solution. α-FeO(OH) is more reactive than α-Fe2O3, but generally converts to the latter at 220–260°, independently of pH, in neutral or alkaline media. Interpretation of the results is complicated by the ion-exchanging properties and varying degrees of crystallinity of M(OH)2, and by the tendency of M(OH)2to decompose to MO or to be oxidized at the higher temperatures. Nevertheless, the order of reactivity M = Co ≥ Mn > Ni > Zn > (Mg, Ca, Cu) in MFe2O4formation is generally valid. The formation of MFe2O4is limited by kinetic rather than thermodynamic factors for the various M listed (except Cu).