The systems NixMn1-xC2O4 center dot 2H(2)O (x=0-11. 0.34) are characterized by XRD, SEM, TG/DTA, EGA-MS and magnetic measurements. The last confirmed that the studied samples are real solid solutions. The SEM reveals that the morphology depends on both the excess of C2O42- and the initial ratio Ni/Mn. The thermal magnetic investigations (in situ) show that: (i)the presence of Ni in NixMn1-xC2O4-2H(2)O leads to decreasing in the decomposition temperature in regard to that of the manganese oxalate; (ii)upon increasing the Ni content the temperature of decomposition (in air) is growing up; (iii) the presence of Ni stabilizes the manganese with respect to oxidation, in spite of the occurring process of decomposition. (C) 2008 Elsevier B.V. All rights reserved.