The thermal behavior and magnetic properties of two manganese(II) oxalates crystallohydrates are studied and compared. MnC2O4.2H(2)0 crystallizes in monoclinic syngony, while the MnC2O4.3H(2)O-in orthorhombic. The difference in the crystal lattice is responsible for the different thermal behavior of the samples. This fact is proved by in situ measurement of the magnetic properties, TGA, DTA, DSC, and X-ray diffraction. The dehydration proceeds in one step with MnC2O4.2H(2)O (AH = 86kJ/mol) and in three steps with MnC2O4.3H(2)O (AH = 132 kJ/mol). The exchange interaction between the manganese ions in MnC2O4.3H(2)O is changing from anti ferromagnetic to ferromagnetic during the entire dehydration process, while in the case of MnC2O4.2H(2)O the same interaction remains almost constantly antiferromagnetic. The two crystallohydrates reveal different initial and final temperatures of dehydration, thus changing the region where the anhydrous oxalate exists. The oxidative decomposition of the anhydrous oxalate leads not only to Mn(III) but also to Mn(IV) and an oxidation process is much weaker when starting from MnC2O4.3H(2)O. This fact shows that the orthorhombic crystal lattice stabilizes the lower oxidation states of manganese. The annealing of the monoclinic MnC2O4.2H(2)O at 450degreesC leads to its complete transformation into Mn3O4 with hausmanite structure. At the same temperature, the orthorhombic MnC2O4.3H(2)O gives not only Mn3O4 but also a great quantity of cubic Mn2O3. (C) 2004 Elsevier B.V. All rights reserved.