Methylamine, diethanolamine, 2,2’-bipyridyl and 1, 10-phenanthroline complexes of Ni(II) were synthesized. Thermal investigations were carried out and the stereochemical changes which occurred during the thermal decomposition were studied. The complexes were characterized using elemental and thermal analyses, IR and electronic spectral data, magnetic moment values and conductivity measurement. They were found to have the compositions [NiL4(H2O)2]X2 . 2H2O, [NiL2’X2] . nH2O, [NiL2"X2] . nH2O and [NiL2"’(NCS)2] where X is Cl- or Br-, L is methylamine (MeA), L’ is 2,2’-bibyridyl (bipy), L" is 1,10-phenanthroline (phen), L"’ is diethanolamine (DEA), and n varies from 0 to 2. The configurational changes of the complexes and their intermediates were studied using thermal analysis (TG and DTA), and IR spectral and effective magnetic moment data. All the complexes of Ni(II), except [NiL2"’(NCS)2], appeared to possess octahedral structure; [NiL2"’(NCS)2] is tetrahedral in structure. Thermodynamic parameters, the activation energy E(a)*, enthalpy change DELTAH, and the entropy change DELTAS, for the dehydration and decomposition reactions of the complexes were evaluated using standard methods. A linear correlation was found between the E(a)* and DELTAS values for the decomposition reactions of the complexes. Probable decomposition mechanisms are also proposed.