The thermochemical changes in the coordination structure of diaquabis(N-substituted ethylenediamine) nickel(II) complexes ([Ni(H2O)(2)(diamine)(2)]X(2)) were investigated by means of TG and DTA, and electronic spectroscopy, where diamine is N-methyl-(N-men), N-ethyl- (N-een), N-isopropyl-(N-ipen), N-phenyl-(N-phen), or N,N-di-n-butylethylenediamine(NN-dben), and X is Cl- or Br-. The diaqua complexes prepared were all trans. The trans complexes with N-mono-substituted ethylenediamines brought about a deaquation-anation on heating, retaining the original trans configurations. Only the complex chloride with NN-dben transformed into the cis-dichloro complex upon thermal dehydration. The mechanism of the trans-to-cis configurational change of the complex chloride with NN-dben was proposed, acid the effects of N-substituent group(s) upon their thermal structural changes were discussed.