The thermal reactions of trans-[Ni(H2O)(2)(bezen)(2)]X-2.nH(2)O (bezen = 1-benzyl-1,2-ethanediamine; X = Cl-, Br- or NO3-; n = 0 for Cl- and Br-, and n = 1 for NO3-) were investigated by means of TG/DTA, DSC and electronic spectroscopy in the solid phase. All the trans-diaqua complexes brought about a deaquation-anation on heating, transforming into the cis-dichloro, cis-dibromo, and cis-nitrato complexes. The latter two cia complexes irreversibly isomerized to trans ones upon further heating. This type of isomerization is rare and so interesting. The enthalpy change of the exothermic cis-to-trans isomerization was -7.16 and -11.8 kJ mol(-1) for the bromide and nitrate, respectively.