The crystal structure of two [XL3](NO3)(2) complexes, where X = Co(II), Ni(II); L = 2,2'-bis-(4,5-dimethylimidazole), were characterized by single crystal X-ray diffraction using SHELX-97. Thermogravimetry (TG) and evolved gas analysis by coupled mass spectrometry (EGA-MS) were used to study the thermal behaviour of both complexes. After dehydration, a multi-step decomposition occurred in the complexes examined due to the release of both the nitrate groups and the three ligand molecules. A final residue of the metal(II) oxides was found at 1073K. A confirmation of the oxidative decomposition pattern initially proposed by the percentage mass loss data was obtained by the evolved gas analysis. Finally, data collected from TG experiments at different heating rates concerning the oxidative decomposition steps were processed using the Kissinger equation to obtain activation energy, pre-exponential factor and rate constant values at the middle of the experimental temperature range, while the dependence of activation energy from the degree of transformation was assessed using the isoconversional method of Ozawa-Flynn-Wall. (C) 2010 Elsevier B.V. All rights reserved.