The aquaorotatotriethanolaminenickel(II) monohydrate, [Ni(HOr)(H2O)(tea)](H2O)-H-. (1), was synthesized and characterized by means of elementel analysis, IR and UV-Vis. spectroscopy, magnetic susceptibility, thermal analysis and X-ray diffraction techniques. The nickel ion in [Ni(C5H2N2O4)(H2O)(N(C2H4OH)(3))] is chelated to the deprotonated N3 pyrimidine atom and to the carboxylate oxygen atom of the bidentate orotate dianion, and to the one nitrogen and two oxygen atoms of the tridentate triethanolamine molecule and its octahedral geometry is completed by an aqua ligand. It crystallizes in the monoclinic system, space group P2(1)/c with lattice parameters a = 7.1528(5) angstrom, b = 19.4903(14) angstrom, c = 11.8085(8) angstrom, beta = 106.237(5)degrees, V = 1580.55(19) angstrom(3), Z = 4. An extensive three dimensional network of O-w-H... O, N-H... O and O-H... O hydrogen bonds, pi-pi and pi-ring interactions are responsible for crystal stabilization. The decomposition reaction take places in the temperature range 20-1000 degrees C in the static air atmosphere. Thermal decomposition of 1 proceeds in three stages.