The novel bis(acesulfamato-O)tetraaquanickel(II) and bis(acesulfamato-N)tetraaquacobalt(II) complexes have been synthesized and characterized by elemental analyses, magnetic measurements, u.v.-vis and FT-IR spectra. The thermal behavior of the complexes was also studied by simultaneous TG, DTG and DTA methods in a static air atmosphere. The crystal structure of bis(acesulfamato-O)tetraaquanickel(II) complex has been identified by single-crystal X-ray diffraction analyses. The acesulfamate ligand (acs) acts as an O-donor through the carbonyl oxygen as a monodentate ligand in the Ni(II) complex. The Ni(II) ion structure resides on a two-fold axis and is coordinated by four aqua ligands defining the basal plane, and by two monodentate acesulfamate ligands occupying the axial positions. The chromotropism of both complexes has been studied using thermal and spectral analysis. The bis(acesulfamato-N)tetraaquacobalt(II)complex is found to be very soluble in water and organic solvents and exhibits a reversible thermochromism from pink to violet depending on deaquation, which occurs in two steps, in the solid state. The bis(acesulfamato-O)tetraaquanickel(II) complex has shown two thermochromic properties one of which is a reversible and changes the color from green to yellow by an endothermic effect, whereas the other one is irreversible and changes from yellow to brown depending on deaquation, in the solid state. The bis(acesulfamato-N)tetraaquacobalt(II) complex exhibits solvatochromism in solvents with different donor number and ionochromism in the presence of various cations in solution.