Complexes of the tumor cell differentiating agent hexamethylene bisacetamide (HMBA) with nickel(II) halides e.g. Ni-2(HMBA)(3)Cl-4 . 2H(2)O, Ni-2(HMBA)(3)Br-4 . H2O, Ni(HMBA)Cl-2 . H2O and Ni-3(HMBA)Cl-6 . 3H(2)O have been synthesized and characterized by elemental analysis. On the basis of their reflectance spectra and magnetic susceptibility data, the complexes with 2:3 stoichiometry are assigned as mixed octahedral-tetrahedral structures and can correctly be formulated as [Ni(HMBA)(3)][NiX4]. nH(2)O (X = Cl; n = 2 and X = Br, n = 1). On the other hand, the complex Ni(HMBA)Cl-2 . H2O has essentially a tetrahedral structure and the complex Ni-3(HMBA)Cl-6 . 3H(2)O a chloride-bridged polymeric octahedral structure.