Cation deficient spinels Ni(x)Mn(3-x)square(3 delta/4)O(4+delta) (0 less than or equal to x less than or equal to 1) have been prepared by thermal decomposition of mixed oxalates Nix/3Mn(3-x/3)(C2O4). nH(2)O in air at 623 K. They have been characterised by temperature programmed reduction (TPR) under H-2, the reaction being followed by gravimetric and powder X-ray diffraction measurements. It has been shown that TPR proceeds in several steps. The first steps correspond to the loss of nonstoichiometric oxygen leading to the formation of a stoichiometric oxide. During the following stages the manganese cations are reduced, causing the spinel structure to be destroyed, and the formation of solid solution of NiO in a cubic MnO. Subsequently, Ni2+ cations undergo a reduction to metallic nickel, and, finally, a mixture of nonstoichiometric MnO1-delta and metallic nickel is formed. These oxides contain a high level of vacancies which vary with the nickel content with a maximum of delta approximate to 1 near x = 0.6. This nonstoichiometry is ascribed both to the presence of Ni3+ and excess of Mn4+.