Nickel manganites have been prepared by thermal decomposition of mixed oxalates Nix/3Mn(3-x/3)C2O4 . nH(2)O in air at 623 K (0 less than or equal to x less than or equal to 1). These oxides are finely divided materials because their BET specific surface areas are close to 100 m(2)/g and the average diameter of their crystallites is about 10 nm. Electron diffraction studies of these compounds show that they crystallize in a spinel-like cubic structure. Thermogravimetric analyses show that these oxides are nonstoichiometric cation deficient spinels Ni(x)Mn(3-x)square(3 delta/4)O(4+delta). delta depends on nickel content and reaches a maximum of 0.9 for x = 0.4.